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Is there an equation or does anyone have a good rule of thumb with respect to determining a crank length for kids? Thanks
There is some good info and a few different formulas mentioned in link below.

I've seen some other BMX calculators too which are kid centric as well.

In general this has worked well for us. Better to error on the shorter side than longer.

16" Bike - around 100mm crank
20" bike - around 125mm
24" bike - 140mm or 135mm

That being said, plenty of kids get along ok on longer cranks even if it's not ideal.
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