"I bring cookies to the girls and the boys."
Translation:Crustula ad puellas et pueros porto.
You would have used crustulum as the accusative singular, not the accusative plural. I would highly recommend using a chart or table that gives you the case endings for the different declensions if you are having trouble remembering all the different endings.
Here is a site that has tables for the five declensions: http://dcc.dickinson.edu/grammar/latin/case-endings-five-declensions