Tea Tree is for fleas, rather than headlice (according the Tisserand original aromatherapy manuals) its actually Rosemary oil that is the aromatherapy remedy head lice treatment & it does work really well as a repellant, & having tested it on one of the nasty little buggers, it WILL kill 1, where tea tree wont, but you'd need bucket full to clear a whole head - so --
you need a "nitty gritty nit comb - others done work half as well & just be very very thorough, with wet combing with conditioner or even mayonaise(suffercate the little buggers), the nitty gritty will remove eggs as well as lice - you need to keep repeating this & if hair is straight, or short you can probably managed with dry combing in between - until you get rid of them all
use Vosene shampoo - they now have a bright green 1 that is for headlice, but the old one used to work too - also use their detangling headlice repellant spray, but add about 20 drops of rosemary oil to it - once the headlice are clear, we find this works brilliantly for keeping them away - & DD has waist length hair :) - plaiting it helps too though
I've a complete blank on the brand name of the only over the counter one that actually works, but it is the same 1 as you get on prescription, its silicon based & suffocates them & then you repeat a week later when eggs have hatched - it is good stuff, though still needs thorough combing - DD hasn't had a problem with her skin reacting to it & she does have very sensitive skin, diagnosed as excema, but turned out to be allergy related urticaria instead, so you may well find your DD is fine with it too- we've used it a couple of times with excellent results - when I've stupidly ran out of rosemary oil & vosene spray - make sure you treat the whole family though