CHICKADEE!!!
Chickadee Not sure where you are located/if it's available internationally but Chickadee is a FANTASTIC Canadian environment/animal/science magazine - 100% grew up on it & owl!
It's supposed to be for 6-9 but there are always lots of bright pictures & fun puzzles you can save for years & years. I'm sure I still have a PILE of them at my parent's house somewhere

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That's such a great idea though!!

I usually just hand my little guy all the junk mail

Edit: there is actually a 'Chirp' magazine for 3-6 I don't know anything about it though

(just hit subscribe on the link & there is all the info)