Sorry, sounds compliated....
DP and I have been together since September. He has 2 kids from a previous relationship and his ex has found out about us. The relationship between these kids and their mum has been difficult for years after she walked out and left them with their dad and became worse after she recently had a new baby. Son aged 11 refuses to go out with his mum and only sees her when she calls to their house. Daughter aged 14 does see her mum and has recently started staying overnight alternate weekends.
Both kids are finding it v difficult despite their parents being divorced for almost 5 years...
I am the first real relationshiop that dp has had since the divorce and the kids have met me.
Problem is that their mum has got wind of it and she seems to be determined to throw a spanner in the works 
The latest plan is that daughter will stay with her mum over xmas and they will visit the son for 1 hour xmas day morning. However, she has now changed the plans and says she refuses to visit her son on xmas day and will see him on 27th when she brings back her daughter... which means that daughter can't see her dad at xmas and son can't see his mum, which despite his refusal to visit her, he will be devastated not to see his mum.
I spoke to dp about it, he says that it has never been a problem before... seems that my relationship with dp may have caused her to change her plans.
Clearly this is none of my business, but how can I support Dp in all of this? He freely admits that he would gladly tell his ex to forget the whole thing and stay away but then that hurts the kids so he can't and won't. It's all such a mess and waste of time all for nothing.
Any advice/experienceS?