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Here's something as a fun reference: A collection of all the adult designs for the kid characters I could find so far. Obviously I may have missed some but I tried to find them all.
There are some things to consider, Neil's adult design looks considerably older than the other characters, he looks like he is in his 40-60s while most of the cast are in their 20-30s. I am not sure if this means Neil just ages incredibly poorly, or we just assume that's how he looks when he is even older, and just imagine him not bald for his younger adult look.
Also, I don't think the Cindy one was meant to be taken seriously. She isn't going to just develop Ursidae facial structure when she gets older. I do think she'd be very hairy though, but still look human.
How do we like the designs so far? Any you like in particular?