I don't think she cares so much that the kids have fun with their dad - she just wants them to have MORE fun with her. AKA love her more. It's a needy chick thing, and probably one of the reasons they are separated in the first place.
Possesiveness over the kids leads to the fear that the kids may start prefering the dad over her.Its also possible that the mum and dad dont get along well and the mum doesnt want the kids to be v.friendly with the dad.
and that's why they don't live together .women have the capacity to as selfish vindictive and cruel as well as anyone , maybe even more .