Sexing all depends on the size of the tortoise and not the age.
At around 4 inches SCL Spur-thighs are sexable 90% of the time. Your best off to wait to see what sex the 2 you have are before you get another tortoise. Also best to have either you or your daughter have 1 male and someone else have 2 or more females. Than you'd just bring them together for breeding and the male can go home afterwards and not be around for him to bother the females constantly. Yes male Spur-thighs tend to fight with each other when kept together.
Danny