It's a big disappointment, and a wasted opportunity that the younger age stages of the Inklings have never officially appeared in one Splatoon series game. The franchise only focuses on the 14-year old Inklings, but it shouldn't be that hard to use the cute newborn, 1 year old. 4-5 year old, and 8 year old ages for the race.
Here's an idea: If there was a Splatoon game released on the Nintendo 3DS (in between Splatoon and Splatoon 2), it would give lots of focus on these stages of the Inklings, and younger Callie and Marie. This focus would show how the Inklings develop, and what they are capable of doing when they are younger. The game would also let you play as Callie and Marie, since the younger Inklings don't have full control over their ink abilities.