I don’t really see the link between toddler and ninja turtles and apart from perhaps sticking to “child-friendly” (i.e. strong, cheerful) colours I think you should target the parents (what do they perceive as being fit for the child) more than the child.
Think about who initiates and influences the buy, who decides, and who actually does the purchase in the end. Apart from being one of the users of the product (and you’ve covered both the child’s and the parent’s needs here…?), the child is perhaps not a very active part in this purchase, except that it might be more drawn to certain colours and thus influence the buyer, if the child is present when the shoe is being bought.
Anyway, as far as the child goes, I don’t think Ninja turtles is the right way to go.
Interesting project to follow, by the way. Thanks for sharing.