If this is for road riding, then I would recommend a 650c tire road bike. Both Cannondale and Trek made a bunch of these 650c bikes from the mid 1990's to the mid 2000's. Another choice is the Felt F95 Jr which was made in the 2010's. This is my son's Felt F95 Jr:
Jamis, Bianchi, Diamondback and Specialized also made some 650c wheeled bikes but they are rarer. The Fuji Ace is another option but it does have low level components and paddle shifters (which might come in handy for some younger kids who have trouble operating brifters)