Boo and Riley are both from the early 2000s, the hair ties were a popular fashion for toddlers back then. Boo wouldn't be Riley's mother because they're hair colors are different. Also, Riley's mom's ACTUAL name is Jill Anderson, while Boo's actual name is Mary (as seen on the right top of her drawing). Boo is around Riley's age, but probably older. In Toy Story 3, there was a child who looked a lot like Boo, and the story is set 2010. It couldn't have been set 20 years later after Monsters Inc. because the playground could have been a recycled background from TS3 to save budget. Theory, busted.