I don't think Sue stressed enough: You will NEED a GAC ...
Handicap placards intended for motor vehicles have no meaning except in the vehicle transporting the owner of the placard.
The handicap signs you can buy in the locations Sue mentioned have absolutely NO standing when used by individuals (vs., say, being posted in front of a parking space). After all, they're available to absolutely anyone with a few dollars. Ditto for the small square HP stickers.
Subject-specific - no sign, sticker, or placard is valid for use inside any Disney park. Simply go to Guest Relations the first day you enter a park and request a "Stroller as wheelchair" GAC. You will need to bring the child with you.