S.E.
It's really important for kids' self-esteem that they have playmates over to their house. They like to "show off" where they live, share their toys, etc. Even if you don't have much to show by adult standards, it doesn't matter to kids. They really like to host their friends.
I don't think you need to be pushy, I think you could just say, "It's so great when you take the boys - it really does give me a break and I appreciate that - but it's important to Billy that he have Stevie over to his house, too. So this week, I'd like to take them here. Billy's really excited to share some of his new toys and show Stevie his room, etc..." I can't imagine her arguing with the desires of a 6 year old, but if she does, I'd just say that it's important for kids to host some of the time, so this week you'd like to do that. Period.