Although I do understand your frustration and know that visitation can get quite inconvenient; especially when the children get older. Like AMom said, talk to your X and see if he would be in agreeance to particpating in allowing the children resume normal activities on his weekends. If not, then see if there is an alternative visitation that he could agree upon so the children can continue with their activities. As the custodial parent, you are obligated to encourage the visitation with the non custodial parent. So in all reality, you cannot stop them from visiting. If you can work something out, and try to cooperate with him regardign the children that is always best. But you always have to encourage and respect the visitation schedule, regardless of what the children or you have planned. Good luck!
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*There is no love, like the unconditional love of a Mother*
-I am not an Attorney, any advice given is strictly advice-