I don't know what the wrt stood for on the first page when you referred to talking to the other side. Do you think you could talk to stepmom? As a mom and stepmom, I love my kids and spending time with them, but I refuse to have them sit in my face all summer long. I doubt seriously that stepmom has the intention of having them cooped up all week with her.
How do the children get along with stepmom? Did your ex mention her being willing to have the children or what her plans might be?
Besides being able to bond with stepmom, have another set of friends, and experience a different summer, they'd probably be talking to their father more, at least by phone (I'm going by the belief that she's not the fairy-tale evil stepmom). If he's starting this job soon, you might want to consider sending the children for spring break to see how that goes and what you can expect.