OSB vs Plywood

If I were you, I'd look no further than LP Smartside.

http://www.lowes.com/pd_55897-132-2...oductId=3058153&N=0&catalogId=10051&langId=-1

Hardi also makes products available in sheet form that hold up well. Little more difficult to work with, but in my opinion, a superior product to the LP. Cost is comparable. If you do decide to go with the plywood, go with 1/2". The cost will be virtually the same, and the added strength will make it last longer.

As it is a child's clubhouse I assume that it doesn't have to win any architectural digest awards; therefore good exterior paint over plywood will give you service for a good number of years depending on the climate. I doubt that the grandchildren will play in that clubhouse though. Consider using an exterior grade plywood at least 3/8", 1/2" is better, or a "rated sheathing". I have never seen 1/4" plywood that is rated for exterior use, in your area you may have exterior 1/4 " available I still don't see it holding up for long. The LP siding (wood particles and resin) and the Hardi-Plank (fibre cement board) both will hold up very well for many years, your grandchildren will play in the clubhouse. Good Luck.

IF you want it light, use plywood. OSB is much heavier.