His wish: A cubby/tree house. Sounds a lot like the one in the Swiss Family Robinson, which we've recently read a very abridged version of. The YouTube clip of the tree house in the 1960 movie blew his mind a bit.
Then we asked him what he thought Mummy & Daddy would wish for. His answers:
Dave: Some wood to build be a cubby house.
Me: A new washing machine to do more washing.
For the record, there's nothing wrong with our current washing machine. Maybe he thinks I'm not keeping up.