Reading, lots and lots of reading both indivudual and out-loud to one of you. Also as the poster above brought up your local library it made me think that they may have some kids literacy programs over the summer. People can find different modes of learning useful, so consider using more than one method to help. 

Also, make sure to have lots of conversations with them so that they can hear you talk properly too and pick up on how you use words. Maybe watch some educational tv programs together and talk about them afterwards (my SD love nature documentaries and science shows so we watch a lot of them).

Keep correcting them but maybe not every sentance they say as that could get fustrating to them, but when you do correct them try to get them to repeat the correct sentance back to you. Thank them when they ask you for something using the correct words. Praise them when they say sentances properly. 

You might also be able to come up with little word games conjugating verbs for silly sentances with them, perhaps around the dinner table. The person who can correctly conjucate the verb with the siliest sentance "wins". They may need quite a lot of help at first. So one person begins the sentance but another finishes it...Last year I went to the poop factory when I fell in the poop, yesterday I was at the poop factory and I saw a giant fart, We are in the poop factory and I can see pink poop, Tomorrow we are going to go to the poop factory to taste poop ice cream. ...encourage their imaginations and sense of fun.

Or perhaps do some story telling games around the table or you can buy various dice/card/board games that encourage the use of language. 

Some kids will want to know the logic behind all of the different forms of the verbs and some will not have the patience to learn it and will learn better by being immersed in the language and using it regularly.

Remember that carrots tend to be a lot more effective than sticks, so keeping things light and fun, using lots of praise, some rewards and being consistent will help a lot.

This definately has to be a whole home effort with Dad involved too, if he is not interested in working on this then there is not much you can do on your own. 

Let your DH handle it. They are not your kids, and their development isn't your problem. I say this as someone who once thought like you.....that I had some sort of role in skids life....it simply isn't so. Trying to fix them won't work out well for you, but I guess it's a lesson we all must learn. hahaha I'm still learning mine......

Is the BM gone or something and kids live with you full time? If you and DH are the only ones around and BM is gone forever then maybe I can see you pitching in.....otherwise....not your circus not your monkeys!!!

BM screw up the kids and then gave up,  you really think you are going to fix this.  10 yo in school should not be making those errors.  No matter how bad BM is.  There something wrong with them. They should be tested to see what the problem is.  You need professional help with them. 

As a SM you will get no thanks from this.  They have a BM and BF you are not one of them 

You don't deprogram it. You immediately stop tolerating it and demand and tolerate only age appropriate behavior.  We landed on this when SS was going through pre visitation behavioral degradation and post visitation behavioral detox after visiting the shallow and polluted end of his gene pool.

He would leave for SpermLand a well behaved little boy who could have an intelligent conversation with a significant vocabulary and come back a screaming, whinnying, crying PITA who would plop down in the middle of the floor, point at the refrigerator and grunt instead of asking for what he wanted.  We just stepped over and around him until he used his words.  If he melted into a tantrum we would swat him on the ass, walk him to a semi-remote corner by an ear, plant his nose in the corner, swat him on the ass again and tell him to stay their either until we came to get him or he could pull his head out and behave like a little boy instead of a baby.   

Initially we attempted to work through it in a progressive manner.  It didn't take long to figure out he would play that out for as long as possible.  So we landed on zero tolerance immediate behavioral compliance.  We took what was about a 10 day nose dive in behavior prior to leaving for SpermLand and about a 2-3week shit storm of behavioral detox when he returned home from SpermClan visitation to a few days.  A few days that he had to learn quickly to reconnect with how life was in the real world of our family or his existence was decidedly unpleasant and significantly separate from the happenings and people in our home. He quickly reconnected and it was much more pleasant for everyone.

Two decades later he is a high performing adult with a successful career and an amazing man of character and capability.  His three younger also out of wedlock Spermidiot spawned half sibs by two other baby mamas who were raised by the SpermClan are incapable of providing for themselves, two of the three are drug using gangbanger wannabe's just like their Spermidiot, and all three of them cannot function out from under the toxic influence of the shallow and polluted end of their gene pool as governed by SpermGrandHag.

Tolerate nothing less than age appropriate behavior.