I've worked on a LOT of kids BMX bikes. Almost all of them used the 29 mm size. Last month I needed to buy a replacement freewheel for my grandson's bike and none of the local shops had a 35 mm freewheel in stock. Lots of 29 mm freewheels.
All that I was really saying to the OP is that it's prudent to measure rather than to guess.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.
Last edited by Retro Grouch; 07-14-15 at 03:00 PM.