I'm not an expert, but I think the best exercise for kids this young is just being a kid! Active kids (i.e. those who spend most of their time playing and not watching tv) shouldn't need to have extra exercise programs because they spend all their time running, jumping, hopping, skipping, and climbing - all of which is great exercise for them. Follow a two year old around on the playground for an hour...you'll be huffing and puffing!
If they have the attention span for it, you could try some playground games like "Red Light Green Light" or "What Time is it Mr. Fox?" to get them running even more. Also there are lots of great gym programs like "Little Gym" and "Gymboree" if you don't feel like your little one has enough opportunity to play outside or on climbing structures.
But I think any kind of "adult exercise program" could be dangerous for their little joints, softer bones, and weaker muscles.
Have fun with your little one!
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