The Octonauts 

The look of the turtle in this show is smooth and looks very friendly, having a small details such as freckles on the cheeks and light blue spots over him. The eyes are also simple, they are pitch black with a small white spots making the creature look cute and innocent, moreover, by look of the creature, the target seem to be younger kids because of the facial expression and the colors given onto the character.

Pokemon

Comparing this look of a turtle to the others, it has bigger, brighter eyes, with a curled tail at the back. It also has a big mouth and two small nostrils. By the look of the design, it seems to be aimed at children a little older then 'The Octonauts', as the show this is from 'Pokemon' is a show about creatures fighting each other, to back this up, the creature looks it could fight, standing up on two legs and has a rounded body. 

T.M.N.T (Teenage Mutant Ninja Turtles)

The style these turtles give is that they mean business, equipped with weapons and bandannas, these turtles aim to look dangerous compared to the other turtles. By just the look, you know each one has their own personalizes, to add, the turtles are humanized as well, standing on two legs and having similar hands to humans. By the look, I can tell this is for kids who are into ninja styled battles and humor. As I know the characters love pizza and have childish sense of behavior, moreover, once again, freckles are introduced once more and are added on the childish turtle in the group, indicating he is the youngest and silliest.

Adventure Time

The design of this turtle is simple and very different from the others. For example, its has no fingers and toes, it has a crown and also has yellow hair. It's eyes are closed and by judging the way it is standing, it could quite old and innocent. The show is stars in 'Adventure Time' is aimed for kids with some sort of maturity, so judging by the look of the character, it goes for a suited look to the other characters, simple but somewhat detailed, such the pattern of the shell on its back and the teeth in it's mouth.