the party is for the kids, not you, and if you throw a party, its not up to anyone to offer to pay for it. if you want him to pay for half, call him and tell him that is what is expected of him if he is to attend. that being said, my sons mom has her own party and i have my own mainly because i dont want to be around her and dont want her around me. gently tell the kids mom how you feel and let her decide. she is the parent and you dont want to step on toes that if you break up she will have to mend alone.