I'm gonna ask the question again, because it sounds like with all the upheaval in both children's lives, it might be needed... Have you looked into family counselling?
Your daughter, it seems now is being put into a position of responsibilty which she might or might not be ready for. Your son, sounds like he's being a bit rebellious, and thinks the girls are ganging up on him (ask any guy who grew up in an all female household, I think that's a common reaction)
Crying because he didn't get a toy he was promised, sounds like an overreaction for a 10 year old, there's something deeper there, perhaps it wasnt the toy but the fact that he was going to spend some time with mom - and that didn't happen... Dad has already left and doesn't seem him every day.. now mom said he was going to get somethign an didn't coem thru... 10 year olds minds work differently...
(or i could completely not know what i'm talking about...
)
When a kid cries, heck when an adult cries, it's usually over frustration... it's rarely about what they say they are crying about. (unless they stub a toe because that hurts like hell)