depends, do you want then to toe as close to the abyss of hell as possible or keep a safe distance from it?
lets presume you want them to keep a safe distance so that if they stumble and fall there is ample room to correct and not descent into hell by one simple misstep
in that case, their hair should accentuate that God made them male or female (and since long hair is said to be an honor to women, im going to assume we are talking about boys here)
so, the best way to answer your question is to have them walk in front of a stranger and ask the stranger if they are a boy or a girl
if the stranger can not tell by their haircut if they are boys, you got a problem
look at Revelation 9:8 for instance, where a description of the locusts is given.
it doesnt tell us how long their hair is, it simply explains they have " hair as the hair of women"
this tells us we can recognize male or female by the hair, so as long as your little once adhere to that they will be fine
all kidding aside though, the length of their hair will not determine their eternal home, the only way they can get access into heaven is by admitting they are sinners and therefor worthy of death, and accepting that Jesus paid for their sins on the cross, and on that merrit alone they can enter heaven
of course, by the same token, the only way they can enter hell is by rejecting Gods offer of salvation