I gave my 8 year old daughter children's plus cough almost 2 hours ago but isn't helping, would it be ok if i gave her tiny bit of adult nyquil?

Unlikely to help: Cough is a protective reflex that allows the lungs to clear itself of irritants & mucous debris. Studies of cough/cold preparations find they do little if anything in kids under 12. Mucinex can thin the secretions so they are easier to cough out, but the antihistamine containing products like NyQuil thicken them, making them harder to move.