Lice "treatments" do not work, the bugs have built up immunity to it.
Shampoo their hair, condition it, comb the tangles out, rinse, leave it pretty wet. Have them lay by you with their head on your lap, you can put a while towel down to protect your lap if you want.
Then section their hair off into small sections, Keep it wet with a spray bottle. The cold water stuns the bugs and they'll be moving very sluggishly. Easy to pick off.
Use a small rat tail comb and go through each section, scalp to end. Any live bugs get pulled off and put into a cup of water, they breath air just like we do and will drown.
Any white nits are nothing. They are empty egg sacks. Nits with a black center is a bug about to be born. They need to be pulled off. Either pluck that single hair or snip it off below the nit or slide the nit down the hair shaft using your fingernails. Just get it off and into the water.
Then when you're done with that section pin it up and move to the next section. You MUST keep the hair almost drippy wet. Lice don't like being wet, it makes them cold, and they can't move very quickly when the hair is wet.
After you go through their hair you can dry their hair normally.
If you are still having lice then you're not vacuuming the car seats or the couch cushions or scalding their hair brushes enough or something. You don't have to boil the sheets and bedding or anything like that but you do need to vacuum as much as possible.