Actually, when I first saw this I thought two things... 1) that it was from as it looks a lot more stylized than your older works. Its a nice change! And 2), once I recognized Swift, that she looked younger. Something about her expression gives her this look of innocence and youth.
I do agree with other comments that if you made their eyes and paws a little larger, and perhaps make their legs seem a bit long for their body that that would help make them look younger too. That weird phase of having to grow into their legs and feet, gangly teenager. If you wanted to make them even younger than that, you could add a little more pudge to them for the puppy fat. Maybe make their features or fur a little more round/softer vs the sharper angles they might have as adults.