You can use adult to refer to grownups. We often use contrast to help children understand the meaning of this word. For example, you can also teach the words baby and child at the same time as adult to help your child learn what the words mean. You could say something like, "Daddy is an adult & Jenny is a baby."
When teaching the sign for adult, teach baby and child as well. Using contrasting signs will help baby reinforce the learning faster.