Hi Ladyhawke,
I was just wondering what has sparked such animosity from them - were you the other woman? Does their mother bad mouth you? Its a shame that the children have been caught up in what sounds like a nasty split. They must feel very hurt and possibly abandoned.
Although the children are older now - they were at very impressionable ages when their dad left - probably the worst time when teenagers are trying to find where they stand and all the angst that goes with it. Blaming you probably eases the pain they feel about their father.
Why do you begrudge the time and money spent on them - your partner is their father and they deserve this time and a bit of spoiling.
It is only one weekend in three and he is seeing his children - not off on a jolly with his mates where anything could happen.
I think you need to relax about it and do use the time to have fun with your own friends and family.