I would recommend letting them be kids as kids, so run around how they want- the only real control of their interests should be not giving them sources of cancer like tablets at age 3. Instill the importance of health and get them swimming lessons, so they can never be worried of water and develop good breathing health. This will affect quality of hiking trips.
I like having small day packs with children in mind. The east german boxy boy is silly on an adult man but for a kid it would hold everything they could imagine, and it costs 10 dollars right now so they can throw it around.
harry potter and a series of unfortunate events as they get a little older
biology (buy sooner than later) and history books too
