Welcome to the Bunnings Workshop community @Daddylewski. It's wonderful to have you join us, and many thanks for your question about installing a cubby house.
The timber, at best, will be H3 treated, which means it's suitable for above-ground use. Currently, you have it in contact with the ground and should expect some level of decay. I'd suggest it might be a good idea to slip some H4-treated Pine sleepers under the cubby and raise it off the ground, keeping it away from moisture.