When a child dresses up they represent themselves as someone else–a mother, a father, a dinosaur, a super-hero!
It’s an important part of their development of ‘transforming’ one thing into another. In this case, they are transforming themselves into another role.
Take a look at a short article from NAEYC Is Dressing Up for Halloween Different Than for Pretend Play? for more on the benefits of this type of pretend play and the slight differences between dress-up play and wearing a ‘costume’
For more information about child development and typical developmental milestones head to our Child Development page