The "Disney Princess™" is a relatively recent innovation in Disney's branding, dating back only to 2000 or so:
The rise of the Disney princesses reads like a fairy tale itself, with Andy Mooney, a former Nike executive, playing the part of prince, riding into the company on a metaphoric white horse in January 2000 to save a consumer-products division whose sales were dropping by as much as 30 percent a year. ...
It was about a month after Mooney’s arrival that the magic struck. That’s when he flew to Phoenix to check out his first “Disney on Ice” show. “Standing in line in the arena, I was surrounded by little girls dressed head to toe as princesses,” he told me last summer in his palatial office, then located in Burbank, and speaking in a rolling Scottish burr. “They weren’t even Disney products. They were generic princess products they’d appended to a Halloween costume. And the light bulb went off. Clearly there was latent demand here. So the next morning I said to my team, ‘O.K., let’s establish standards and a color palette and talk to licensees and get as much product out there as we possibly can that allows these girls to do what they’re doing anyway: projecting themselves into the characters from the classic movies.’”
Mooney picked a mix of old and new heroines to wear the Pantone pink No. 241 corona: Cinderella, Sleeping Beauty, Snow White, Ariel, Belle, Jasmine, Mulan and Pocahontas. It was the first time Disney marketed characters separately from a film’s release, let alone lumped together those from different stories.
(from "What's Wrong With Cinderella?", New York Times, December 24, 2006)
Tellingly, The Princess and the Frog was the first movie with a human female protagonist to be developed by Walt Disney Animation Studios after this re-branding took place. (Lilo & Stitch was released in 2002, but presumably it had been in development for some time before then, and was less likely to be influenced by a new marketing strategy adopted while it was being made.) So it seems pretty likely that by showing these new characters as children, they are encouraging girls (i.e., their target demographic) to identify themselves with the characters, which increases their desire for products based off of these same characters.
(As an aside: technically, Elsa and Anna are not Disney Princesses™. Yet.)