School Supplies?

For those that have two families how do you decide who buys what for the new school year? Do both families take part in this or is it only the one that gets the child during the week?

I've seen this go both ways, in fact my cousin is currently going through this. Her ex just bought all the items on the school list. Since she recently had a baby she was relieved she didn't need to go out shopping. Course when he brought her home after his weekend visit he told her their child is only to use them at his house. Problem is he only gets her on the weekends. Any advice for her?