Disney Fact #152:
While there are some interesting theories circulating about Boo from Monster’s Inc., and her potential time-jumping abilities (see: The Pixar Theory), one assumption can be made. Boo and Darla, from Finding Nemo, must have been born around the same time.
The same mobile found in Boo’s room is also seen in Dr. P. Sherman’s office, and while Darla is older than Boo, it’s only by a couple of years.
I like to think that this mobile lulled the hyperactive child to sleep, and then she donated it to her loving Uncle to serve two purposes: calm nervous children, and add to his love of sea-life.