yes the nits & eggs are rock hard & impermeable, the headlice can still be killed but who wants to repeatedly put expensive chemicals on their childs head?
You have to build it into the bath/bedtime routine.
I used to wash & condition their hair in the bath & comb through the conditioner first with a regular comb then a nit comb, could get hundreds out that way.
Then get a DVD on, give them something to nibble ( I had to resort to chocolate buttons then when we were in the routine swapped it for fruit chunks!
)
Comb through each bit of the hair as RT says sectioning as you go using a nit comb. Sit behind them & comb through with a bowl of water, so you can see how many you are getting.
If there are loads do it daily/every other day at least then when they seem to be lessening or gone keep doing it for at least 12 days preferably 2 weeks.
I'd suggest doing it every 3 days regardless of the current state of the scalp. The conditioner helps keep the shaft of the hair slippy so the eggs are less likely to attach & therefore are easier to comb out.
Tea-tree oil is a great preventative, but I'd put it in the conditioner not neat as it'll go further.
No easy or quick solution which is why they are such a problem as many mums can't be arsed I'm afraid!