On a holiday overseas with the family along with her husband. 
she had promised her youngest this and that items without checking they're even sold there. Turns out not available and she was moody and sulky and rude that day. 
 

my inlaws were just pissed off because she killed the happy mood and her idiot husband just smiles like an idiot. The so called presents were for tantrumy daughter who was 4 yrs old and off the charts a littly psycho woth behavioural issues

my sil said that niece was an only child for so many yrs till her sister was born that she's so used to having her ways. She promised all kinds of crap to shut her tantrumy kid down because when she loses it, its world war 3 and that 4 yr old smacking babies and scratching newborns faces and eyes while mummy and daddy turn a blind eye or laugh at what she did.

one time hubby shouted at that grand niece and only then did the mum try to address the issues. 
 

your kids will learn not to depend on daddy because he over sells and over promises. 

When my divorce as final I took my bios on a trip.  Ex took them for dinner the night before we left and told them he was never getting married again.  We get back a week later and they find out he got married while we were gone. Talk about drama!   
 

Ex's are ex's for a reason.   It stings when your bad choice is so obvious and you can't put it behind you.   Use it as a teaching moment about integrity , character traits and values.  What kind of person are you going to be?  

"He has just returned from a 3 week overseas business trip. He was away for our daughter's birthday which really upset her. I'd told him before he went that it would. He didn't care. But what makes this trip so much worse - he told both kids he'd be bringing back "lots of gifts". DD17 got a pair of Nike pro shorts. DS15, a cap. Both are upset. I get that it's stupid material shit but why make promises that you have no intention of fulfilling?"

Ok, I honestly think you are overreacting and showing some typical bm tendencies we deal with on the board.

Your ex is on a BUSINESS TRIPS, sometimes this happens in nuclear homes too and sometimes bdays are missed.  I remember the drama our bm would stir up EVERY time we were out of time and saying how much it would 'upset' sd- if we were away on his bday, or first or last or middle soccer game, any reason.  The guy is working, I imagine paying CS.  He can celebrate on a different date when he's back.  Kids of divorced homes learn to get two of everything and celebrating 'ocassions' on different dates/weekends.  Part of life

He did bring them back something, they reason he likely over-promises is there is a good possibility they only value him for what he brings them (the ATM dad) which BM"s often encourage willing or unwillingly.  I'd take a hard look, what he got them is pretty nice.  There's no reason he has to bring back loads of presents every time he's away on business.  

I would take a hard look at yourself in this dynamic, because I can smell the drama BM from a mile away on that post, sorry, that's JMO.

He doesn't owe them tons of gifts or a key ring every single time.  It's possible he's realizing he's been a disney/atm dad for years and is going to try a different approach to see if they value him for DAD or simply what he buys/gets them.  It's easy for them to get stuck in that cycle of over-gifting.  Again, I think bm's tend to much at blame for this dynamic.

Maybe your kids are spoiled a tad.  Your daughter needs to get over her working dad not being there for her bday and realize she's not the center of the world and people work for a living, and sometimes that means celebrating on different days.  

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Being a current and ex military wife, we never had to option for dates when it comes to travel, not sure if you ex did or not, but I w would just let that part of this go.

However, as a mom, I get being hurt for your kids. Yes, it was material things and I am sure you are trying to teach your kids there is more to life than the gifts people bring us- but also it is a lesson in trust with their dad. He made a promise and failed. 

I wouldn't discuss this beyond- "I'm sorry that things didn't work out, I'm sure he tried". True or not, you can't trash him. Even when I was little and my bio dad was in prison, my parents (stepdad adopted me) "He loves you very much, he is just sick". You have to keep your feelings out of it.

It always amazes me how people get upset because they appear to not adjust their job commitments. That job pays CS.  I suppose you could just say no to your boss and lose your job.  CS doesn't get paid cuz you know, if there's nothing coming in , nothing goes out.   It's logic. If you won't teach your kids about how paychecks are earned , you get a mindset that's warped.   I don't how many times I've seen on these forums BM 's that use this work /family choice to PAS kids into thinking that Dad doesn't love you because he won't be here for event.  That's just crap.  He's working.  Making money to pay CS.    Stepping soap box now.  

I agree with the idea that as a business trip.. it's pretty unlikely that he had any choice in the timing of it.. even if there is optionality to attending business trips.. declining important events can limit your career.. It's not that he doesn't care that she is upset.. it's actually a little sad that at 17 years old she doesn't understand that the obligation for him to attend a WORK even so that he can earn money that supports HER may trump the timing of her birthday celebration.  It's actually unfair to even put that guilt trip ON him.. and at her age.. she should know better.. and if it was you relaying that.. well.. it really was unfair as he just can't control his work trip timing in most situations.

Now.. I can get behind the "over promising" thing.. though it sounds like he did come home with a couple of nice things for the kids.. and at their age (again.. they aren't 5.. they are old enough to not feel totally entitled to business trip gifts).. they seem fairly appropriate.  I'm not sure exactly what they were expecting.. what he specifically told them.  But yeah.. if he was sending pictures home of diamond rings and mountain bikes.. and he shows up with a cap and shorts?  and he wasn't just joking.. as in I send my SD a picture of a lambo and ask her what color she wants.. when she knows full well I am NOT buying her a lambo it clearly is a joke... just being funny.  That's not great.. but I still think what JMTB said is all you can do.. basically.. "I'm sure he thought they what he got you was nice.. or did the best he could.. etc"  

And.. I get that you don't like seeing your kids disappointed.. BUT.. if this is how he has been their whole lives.. I don't really understand why they are still so overwraught over what should be clearly predictable.  I mean.. my SD's BM was the queen of promising things and not following through.. by the time the girls were your kid's age.. they didn't take anything to the bank until it was in their hands..haha.  Note.. they still love their mom.. but they understand that her want to do things and promise things.. are likely that she WANTS to do those things for her kids.. but is just incapable of getting them done in the end.

I always would rather underpromise and overdeliver.. but some people have the best of intentions and just can't get the ball over the line.