The Wacky Adventures of Ronald McDonald: Birthday World (2001)

It's Ronald's birthday and Hamburglar, upset that he didn't get Ronald a present, invites everyone to a fake amusement park known as "Birthday World" to celebrate, run by an evil mad scientist named Professor Thaddeus J. Pinchworm. Everyone is amused at first, but they can no longer escape and no longer fall for the poor designs of the rides until they ride a rollercoaster which turns them into babies (both Franklin and Tika are wearing diapers) and they need to find out how to return to their original ages, even though their certain abilities are limited since they are toddlers.