Op, we have a similar situation.
Dh's ex has had the kids every christmas, for nearly 10 years now. He gets them boxing day when she's done all the family stuff and wants to go out drinking.
Thing is, it can be pretty impossible, short of going round there christmas eve and forcing the kids to leave. Even if you get a court order, there's rarely a consequence for not following it, and as you say, lots of money (which we don't have) in solicitors.
Boom Boom, what do you suggest we do? Can't afford to force it legally, ex won't budge. How exactly do we fight? Go and grab them?
The ex has a big family and there are lots (and I mean lots- those piles and piles you see on Facebook) of presents. We only do small stocking fillers and a couple of bigger things for each child, plus mum pulls out the teary waterworks so the kids are guilted into staying.