What do I say to a 10 year old whose mother doesn't give a s**t about him?

All you can do is be there for them,you need to be the rock in which they can re-lie upon,unfortunately my nice went down the same route,she has an 11 year old girl but has never had custody of her,my sister has that.We are quite a big family so we all chip in and take care of her,she asks about her mum alot and now that shes old enough to understand my sister has told her about her mum.But these kids are still young and i think you should be very careful about telling them anything,i wish you good luck with this,your a caring soul and the kids seem to be in good hands,better that than the alternative.hope you have a wonderful Christmas and make it special for the kids,that way they wont even miss their mum,good luck,Matt.