Quoting Buffy♥:" My TWO YEAR OLD got them from her cousins...twice. It was a freaking nightmare to get ride of them because ... [snip!] ... because she did not want to cooperate with rinsing the shampoo or the combing. Can you use that treeoil on a younger child?"
Generally, yes, as long as it's diluted well in shampoo or whatever. Do a patch test with the shampoo first to make sure she doesn't react. If her skin gets red and irritated, dilute it more and try again. If she still seems sensitive, maybe skip it then, but it should be fine.