If they're willing to install whatever to fix, ask them to install a lot more soundproofing, move you to another unit at no cost increase, or let you off your lease penalty free.

It sucks on both sides because the parents have got to be exhausted, but they signed up for this by having a kid and you didn't; their burdens and misfortune should not be imposed on you.

Also, a kid crying non-stop all day and night is not normal; they should be asleep most of the time. If it's not stopping at all, call CPS to do a welfare check.