Timber Windows for Cubby House

I'm working on a cubby house in the woods and want to make some timber-framed windows. What timber would you recommend that would be most weather resistant, so that they don't warp and deteriorate completely and so the kids can still open them a few years from now?
I appreciate they'll need to be re-treated regularly, which can easily be done.
I just want the timber itself to retain its integrity until the kids have outgrown the cubby house.
Cheers!

I would have thought any sound timber would suit your purpose if it's "re-treated regularly". Both WRC and Paulownia are timbers used by manufactures to make wooden windows that will last a long time.
Whitewood