My girlfriend can't remember the title. She says it's a cartoon about a boy who has a magic treehouse in his back yard and when he goes in it, he is in another world full of creatures, not scary creatures, but yeah creatures. Can anyone help me with a title please??

I am pretty sure it was a movie called "The Phantom Treehouse."

For more info see: The Phantom Treehouse (1984) (TV)

I hope that helps!