The problem is (having had nits three times as an adult!)..
People don't understand the nit life cycle and keep getting reinfected from the same source.
There is no need to treat your whole house either, they don't live off the human head for very long (24 hours at most) so as long as you put clean bedding on after treatment and either avoid places heads rest (backs of fabric sofas for example MIGHT sustain a live louse for a while) for 24 hours or steam treat them, that'll take care of household stuff.
Nits hatch between 6 and 9 days so yes if you treat once and look again even 24 hours later, you'll likely see newly hatched nits - so then folk think 'oh well the nit treatment didn't work...' and give up (and that is how nits end up resistant!)
Treat again.
Every day after treatment comb whilst the hair is wet with conditioner, you can rinse the comb under a tap or dunk it in a jug of warm soapy water (soapy so that the nits sink) to rinse off any nits or lice.
Every day kid goes to school, keep hair tied back tightly or short, and use a teatree based repellant spray on it too (lavender is good as is neem oil!)
Treat again after a week.
Keep up the combing and the repellent spray until child is old enough to keep their head away from their mates!
In my case, repellent spray and combing with a lavendar oil based conditioner kept me nit free until my brain saw sense and left the boyfriend who had given me them three times (I was young, you have to learn this shit!) - even though he remained absolutely riddled with them!