Just thinking ahead here! Is there anywhere you can buy a wooden playhouse, child sized, for outside in the garden? Are they expensive? Is it possible to build one yourself? Is it difficult?
i'm going to just put up a small shed. paint it in bright colours... put some cheap lino in it.... perhaps some rough plasterboard inside.... then put up a decent sized blackboard.. an old telephone that doesn't work and voila!
Judging by my experience you could save yourself some money and get a large cardboard box put a few cushions and a rug in and children would like it just as much. OK it won't survive the rain, but if you have the room bring it in and out or get a new one.