How can I build an outdoor playhouse for my grand daughter, age 5?

I had thought about maybe using wood pallets for walls to make it tall enough for her, have some cheap paneling I could line inside walls. Not sure about how to surface the outside walls cheaply or how to build a roof. She is autistic and has to have all her toys together in once place and her bedroom is not big enough. Maybe pallets could also be used to construct a roof. Any other suggestions would be very helpful and appreciated.

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