K.B.
I don't have any suggestions for a small playhouse but we've had our Rainbow playset for 4 years and my kids still love to go out and play on it. My oldest 2 were about 3 1/2 and 18 months when we bought it. It took the youngest a few months to learn how to climb up into the clubhouse but they've been using it ever since. My now 18 month old can climb up the ladder and the slide to get to the clubhouse. I still have to stand behind him to make sure he gets up there safely, but he's doing it already!! So I now have a playset that can be used by my 7 yr old, 5 yr old and 18 month old!
Rainbows can be expensive depending on the size and features you choose, but they last a long time and I'm told you can actually get a pretty good amount back in resale when your kids do outgrow it. http://www.rainbowplay.com/
Good luck,
K.