I have different experiences with children than Gnosis. My youngest is 7. We just bought him his second bike, this one a hardtail MTB one by 7 I believe. On our last ride I was hoping for a recovery ride and he kept the pace around 15 mph for 8 or 9 miles. I do believe he would have gone longer had I let him. Kids are just like adults in that the variety is so great that forming conclusions is fool hardy. Some kids are strong riders some are not. Some clydes are strong riders some are not.
What I think Gnosis is trying to say however is match the ride to the kids abilities and if it is less than yours go at their pace and distance. I can and do agree with that.
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Sir Mark, Knight of Sufferlandria