B.,
It sounds like you already have a good plan for how to extricate yourself from this situation. Your impending move to Renton is a great opportunity to "split up" with the in-laws. Just be open with them about it and explain your reasons for needing to do this (for your own family) and let them know you'd be happy to help them find another place (this will make it clear that you don't intend to have them move with you, without having to say it directly). If they don't get the message, you may have to use some of the other advice to give reasons why they can't come along.
I think you'll be much happier in Renton - as mentioned before - the schools in N. Renton are great and you'll be close to so many great places and activities in the Bellevue and Seattle area like the KidsQuest museum in Bellevue, the aquarium and zoo in Seattle, the Pacific Science Center, Seattle Children's museum, etc.
You may also want to find a mom's group when you move - such as MOPS (in many area churches) to make new friends in your new area and expand your network! This is how I've dealt with being far from family after moving here from the east coast two years ago!
Best of luck to you-
-K.