Ask him questions, give advice. Solicited or not.  As partners, you do not have to bite your tongue when you see him going down the same coddle rat hole with Ms Kidult Pouty Pants.

 can't stand the complete lack of control I have on decisions re. SD and SS. DH is a total chicken-!!** to say No... when they want to visit they have control, he doesn't say that's not a good time to come

The Disneyland dad I used to live with did this and it was infuriating to say the least. 
 

Had a court order but never followed it, had weekends but never would tell me what time he was picking up or dropping off and any revision were never discussed with me 

Sometimes those kids would appear without my knowledge and sometimes (especially during summers) they'd determine the length of their stay without my knowledge/consultation/agreement "oh I know Initially said little Timmy would be going back to his Mother's yesterday but he wanted to stay another day" 

Anytime I called my supposed partner out on his inconsiderate behavior I got gaslit and yelled at. 
 

It was as if he assumed I had no feelings or could handle being disappointed as an adult (which I can but I didn't want to consistently have my feelings/wants/needs trampled over) and rightfully telling kids NO/Wait/Let me check/etc would send them into major depressive suicide watch or something.  
 

Started to feel like his exwife and kids who contributed nothing to my home (except chaos and drained resources) had complete control over my home every Thursday-Monday/every holiday. 

I ended up losing ALL desire for him because of his lack of boundaries 
 

He needs to be more afraid of pissing you off/losing you than the skids.  How you make that clear to him is the question.  

Can you interrupt him when he's on the phone caving? Like nodding your head NO and mouthing the word? What have your conversations with him about this been like? 

You have your daily / weekly / monthly plans. SK dies not effect them.   If SK show just show up, has nothing to do with your plans to go out. DH can hire a babysitter or stay home.  Your not cooking tonight, not your problem.  You are buying a personal pizza .  For yourself 

I would ask him to put them on hold for a moment or call them back.  Level with him and tell him you don't agree with whatever is being planned.  If that isn't enough, I think booking a spa day and making yourself completely unavailable to him is a reasonable accommodation.