If you have the time to do some of the area off I-5 south of Portland, the Evergreen Aviation Museum at McMinnville has a water park that is very unique...the slides come out of a 747 that is perched on top of the building. The entire museum is a great way to spend a day, depending on how old the children are.
You didn't say whether you would be returning north n the Coast Highway, but this detour off the main highway is well worth your time if you are in the area. There are a couple of nice RV parks in the area, too.
