Why not do it just after your child sleep? Yes, sometimes the bably will cry in the middle of your action, just don't get frustrated.
How old is your child? I would suggest in long run you should plan to sleep with your husband, not your child - I do think seperate sleep do more damage than just the sex issue.
Yes, it's hard - your may have to feed the boy e.g. at 23:00, 3:00 and 7:00 plus some extra.
努力!!