Don't use baby talk or a baby voice when you are talking to the students. It works against you in various ways. Children may feel you are talking down to them and be insulted. Also it is counterproductive. You can't tell children they need to behave properly like big boys and girls and then talk to them like babies - it doesn't make sense and you end up giving mixed messages to the children. Just use your normal voice it is probably easier for the children to understand anyway.