She seems to have a lot of wants and has decided to threaten you with not bringing her children as often. She sounds like a real sweetheart. As the others have so wisely mentioned, remind her of her contract and your policies. I think Amandakdt said it perfectly.
You deserve your holidays and to be treated with respect.
On a side note - I try and get my holiday schedule out by the end of February, usually earlier, so then they can work with my dates.