Talk to your L but I'd say mixedintherut is correct.
X can try to pull whatever crap she likes - she won't have a leg to stand on.
It sounds like a whole bunch of unnecessary work to me. If you needed to you could quite easily account for all of the expenditure and the some.
Separating it could actually harm you if she does try to pull something because it will either show a surplus because you spend your other money on the kids (see, he doesn't need that much!) OR it is in deficit and you're supplementing from your own money (see, he doesn't need more CS because he can afford it himself!!).
So if her income increases dramatically or if yours decreases dramatically she has proof that you didn't really 'need' that much CS.
Never mind that you don't know if she'll only pay for few months or years and then go deadbeat again. The relationship with this guy will fizzle and she'll purposefully not work just to avoid CS. Maybe she'll end up supporting the next guy and his 4 kids by 4 different women.
Who knows? See where I'm going here?
IMHO it also gives her too much free real-estate in your head. The court has ruled ed that she is to pay you $X. The court has not ruled that you need to account for every cent NOR that every cent ONLY be spent directly on the kids. There are lots of indirect expenses too.
It is not her money. It is CS she has been ordered to pay you. It is your money.