Artwork/characters for my swimming school certificates and awards

I run a swimming school and I am in the process of designing our swimming programme for our children lessons, I am looking at getting six characters designed for each level, I would like them to be anthropomorphic, wearing all different swimming gear quite cool looking not cheesy, I want them to have a similar aesthetic as the marvel characters, colourful without being scary. The different levels are Frog, Octopus, Sea lion, Penguin, Crocodile and shark. I have attached the image of the Avengers for rendering purposes and an image I found of a shark wearing shorts, obviously I wouldn't want him/her to be too scary looking and I would like three to be female and three to be male.