I like all the late model shimano geared hubs. The 7 speed is harder to find and the coaster brake on them is weak compared to the 3 speed so you would need a front brake. The 4 speed is not easy to find as are shifters and parts for them. Fortunately all these hubs are well made and seem to do lots of miles with minimal maintenance.
Some of the 3 speed nexus hubs have 127mm wide spacing. Most steel frames can be widened easily to fit any of the geared hubs. Check out how to do it on the Sheldon Brown site.
Buying or building a set of wheels with a geared hub can be expensive. Look for a cheap barely used Schwinn or Electra ladies bike. I have bought them for as little as $30 with 3 speed nexus wheel sets and most were barely ridden. Never had a dud one either.
Also look for a bike with a natural alloy finish wheel set as a painted rim with a front brake can squeal.
These are some of the bikes I have bought for their wheel sets and a few bikes I have built.
Your kid is going to be the coolest kid on the block with that Rollfast!
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Built this one with a 3 speed wheel set which cost $40
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Old electra bought for $60 for the rare 4 speed nexus, like new.
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Used the 4 speed nexus wheel set from the above bike on this trek cruiser.
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68 Typhoon with 7 speed/Araya wheel set. I purchased the wheel set for $10 on facebook marketplace!
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Another cheap lady’s electra i just bought for USD$30/Aussie$$50 for the 3 speed wheel set. Look at the front wheel to see if they have marks from the pads. The front wheel on this one is like new, a sign it was barely ridden.
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My 51 Panther with a wheel set from a late model Schwinn heavy duti. No gears but the wheel set has a Shimano coaster with thick 12ga stainless spokes.