If you've already tried sleep hygiene (routine, winding down before bed, all that sort of stuff, it pops up if you google it) i think you may want to talk to your GP about it. Sleep disorders can start very early and if its consistently taking your six year old two hours to fall asleep, that seems like a bit of a red flag that there's something more going on beyond habit. My oldest has sleep issues and a cup of chamomile tea before bed has helped her a lot.