How tall do 10 year olds have to be?

They don't have to be any height. Every person grows at a different rate, especially between the ages of 9 and 13. Girls tend to be taller than boys at 10.

A US government survey in 2002 listed the average height for US boys as 55.7 inches (141.5 cm) and for US girls 56.4 inches (143.2 cm).