I've used them for me, but don't have experience with them with a child... but for me they were wonderful.
I found they even worked if I took them out when we were up, and then put them back in before we began decent (even though they recommend keeping them in for the entire flight). I did this on a long flight because after a few hours they did start to hurt my ears, so I took them out and then just put them back in about 45 min prior to landing.