Hi T.!
Williams is our family favorite! You've got great resposes already, so I'll have to check back to get some new ideas for myself. Here are some other suggestion:
Kaibab Lake and/or Dogtown Lake have wonderful camping & fishing. We bought an inflatable boat & Daddy took the kids out on the lake...They loved it!
There is a nearby KOA which has a pool & other child related activities.
Williams always has festivities, parades, rodeo's, art shows, etc. Here is an address to check out events that might be happening: www.williamschamber.com
Don't forget to check out the cute little ice cream parlors!
There is a tiny, tiny park next to the skate park that you can also check out.
Maybe you can even check out slide park in Sadona, before or after your trip.
Have a great time! Watch your speed driving through Williams. It's a huge speed trap!