Zack has had a kid version of himself before Casey Universe. This is because his age matched mine during his initial creation, which is why he much more older in the 2nd generation. But that kid version was never added to the canon.

Anyway, I made a new kid version last year and decided to refresh it now as a model sheet for the series. In the original picture, he lacked a tail (human error) and pants. The pants were added since Zack is by no means Aidan.