When I was teaching infants I used to get nits all the time. I hated the chemicals, as they made my eyes sting and my chest tight, so I resorted to conditioner wet combing. Did it every time I washed my hair, and sometimes on dry hair (though it is well conditioned and straight so didn't hurt or cause too much damage - I'm sure it's not recommneded by hairdressers!). Always did it over the edge of the bath, or white paper, or white bed sheet (when dry combing) so I could see exactly what was coming out. I used to get tiny tiny ones that way, and I did always get rid of them in the end.Used a metal comb, not the plastic ones.
Labourious, pain in the butt, but it works, and I always got rid of them. I'm not sure how this would be doing it on a child, rather than yourself, so sorry if inappropriate.
Parents used to swear by the electrical zapper combs - didn't ever use one myself, but others may have experience of these....
Remember that the lifecycle is about 3-4 weeks from one big 'un laying eggs, to eggs hatching amd growing big enough to lay eggs themselves, so you do have to keep going for a while once infected....Miss one tiny one that's just hatched and in 2-3 weeks you are crawling again.
You have my sympathies HTH