I use an external frame when I bring a small kid, because I can carry more.

By the way, when my oldest son was 4, I bought a Kelty Starfish. It was one of the few packs that fit him. Now his little brother, who is 4, will be using it this summer. It doesn't hold much, and it doesn't have a frame, but it holds about what I want to give a 4 year old.

Always remember the one rule with kids. Whatever you give them to carry, be prepared to carry it yourself.
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