My 4 year old has a balance bike and does well on it. He has a peddle-bike but hasn't mastered the actual peddling part.
He is still under the weight limit for his bike seat but even as just over 40 pounds and short for his age, his feet get in the way and he really throws off the balance of the bike. This is a little worse since he can "ride" now and we don't always lean-in together.
In the past we have mostly used his trailer and he has LOVED it. We couldn't sell it to him this spring with free cookies and Skylander stickers. It's a little wide for main roads and very low-sitting.
New this year is the trail-a-bike. It is difficult to mount onto the seat-post bracket and at 40 bucks each we only have one (on my bike of course). He loves that he gets to peddle (he's a big "helper" kind of a kid) and this is now his preferred method of transport. He sits up a little higher. I feel that he is more visible. However, he's in charge of keeping his own balance back there and I can't see him. there is nothing holding him on and I know from third-party reports, that he doesn't always keep both hands on the bars.
Hope that helps.
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"I'm built like a marine mammal. I love the cold! "-Cosmoline
"MTBing is cheap compared to any motorsport I've done. It's very expensive compared to jogging."-ColinL
Rides:
1980ish Raleigh Marathon (Vintage Steel)
2006 Trek 820 (Captain Amazing)
2010 Specialized Tricross (Back in Black)
2008 Specialized Roubaix