This looks like a great play house. I have modified the plans to allow it to suit a higher age group as well.
How to Build a Deluxe Playhouse from DIYnetwork.com
12 Free DIY Playhouse Plans – Thrill your favorite child with a new playhouse that you build yourself. Choose from a variety designs and then download free plans and building guides.
FREE DIY plans for CHILDREN'S PLAYHOUSE are at ana-white.com. Try to pick up materials free from Craigslist or www.freecycle.org etc! Easy to build & very sturdy, yet inexpensive & well planned.
another pallet playhouse with instructions - as well as several other pallet upcycle ideas