You are not going to know until after a test ride. Most kids are a bit nervous about riding a different bike. I've discovered it is helpful to start them out with the seat lower than ideal so they can easily get at least one (if not both) feet on the ground. Each week, I raise the seat a half inch, until the seat is at the "ideal" height. That is the height where their knee is just slightly bent when the pedal is at 6 o'clock.
So, on "Day One", a kid's bike that fits well needs to provide a range of seat positions, from the low position that permits putting one foot flat on the ground, to the "ideal" position that allows the leg to be almost straight when the pedal is at 6 o'clock.
Oddly, many kid's bikes do not permit that range of seat positions. Some allow the low position, but not the higher position. Some allow the high position, but not the low postion.
Electra has begun selling kid sized "Townie" bikes to address this problem. The Townie design makes it easy to get both feet down flat on the pavement with the seat in the position that provides full leg extension while pedaling. It is amazing that kid's bikes have been sold for about 120 years, and Electra was the first company to fully address the needs of the five year old cyclist.