It takes 2 weeks to get rid of head lice, no matter what you do.
I found shampooing with Dr. Bronners soap kills the lice. Then you need to remove eggs and nits.
Get a lice comb -
every day, for two weeks lather their hair with conditioner and comb through completely - this will loosen and remove the nits.
Also, if their hair is long, a hot flat iron will kill nits as well - but you still have to comb it out, and get anything closer to the scalp with other methods.