Is effect of a bad sleep(snoring, nightmares, ) on child brain development reversible?

Yes: I am answered yes only because snoring and occasional nightmares have absolutely no effect on childhood brain development. Constant loss or lack of sleep can affect everyone in different ways. I do not think that it really affects development, long term.

Yes if treated soon: Please have and ent( ear nose throat ) speciallist check your child and have them remove adenoid and tonsil.