Are you taking steps to divorce or are you legally separated?
Regardless, your 15yo is of an age to choose when/how often they see/stay with their df and your 11yo's wishes will be taken into account in the unlikely event that the matter of contact with their df is put before a court of law.
As your h seems to be an unprincipled bellend with regard to promises he's made in respect of the dc, it's possible that he will bluff and bluster claim that he wants have a 50/50 arrangement in order to reduce his child maintenance payments.
If this should be the case you're best advised to sit back, relax, and let him get on with it as, before he can go to court, he must first attend mediation at which time you will be able to express your views and those of the dc.
If no agreement can be reached at mediation and the matter goes to court you can ask for a CAFCASS worker to report on the dc's wishes at which time it will become abundantly clear that, while they are perhaps not averse to staying with their dgm on occasion, they want quality time with their df and would rather be with their dm, and/or engaging in social activities with their friends, instead of twiddling their thumbs at granny's while he's squiring his various girlfriends around town.
In short if he tries this hackneyed cheap trick he stands to make a complete prat of himself and, as mediation and court proceedings don't come cheap, he may end up spending more than he stands to save on maintenance if he were to be successful - which, from what you've said, he very definitely won't be 
Please don't worry about instructing solicitors you can ill afford as, again from what you've said about the dc's wishes, I have every confidence that, should it be necessary, you can successfully represent yourself in a matter such as this.
I would suggest that you tell your dc that if they don't wish to stay with their dgm while their df is otherwise engaged with his latest squeeze tomorrow you'll pick them up, or make suitable arrangements for them to travel home together.
I also suggest you inform your h accordingly and make it clear to him that contact cannot be facilitated when he's not around to interact with and care for the dc.