Unlike adult bikes, which are sized by frame measurements, kids' bikes are sized by wheel diameter — a simpler system that roughly tracks height and age as a child grows.
As a general guide: 12-inch wheels suit the youngest riders, roughly 2-4 years and under 100cm tall; 16-inch wheels suit around 4-6 years; 20-inch wheels suit around 6-9 years; and 24-inch wheels suit around 9-12 years, often overlapping into smaller adult frames for taller pre-teens.
These bands are a starting point, not a strict rule — a standover check with the child actually sitting on the bike is worth more than going by age alone, since growth rates vary a lot between children. Buying a size a child will "grow into" is generally the wrong call: a bike that's too big is genuinely harder and less safe to learn balance and braking on.