You do have a good point there, even though I think it could be better put without the anger, but I still see where you're coming from. However, I believe that the main reason why Otis, the male cow from Back in the Barnyard, has udders when he's clearly a male cow is because they wouldn't be able to show a male cow's body part on a children's show. Just wanted to point that out for you.
That's how I always thought of it too. Considering that the show is focused on Pokemon, the animals that there based on, like birds, fish, monkeys, etc., are mentioned only through the Pokedex when its describing what kind of Pokemon they are.
As for the topic, I think one reason why animals are used for young children shows is because some kids may enjoy seeing animals more than kids their own age having the adventures. Of course, that obviously doesn't work in every case since there is Dora to consider as well, but that's the only currently popular pre-school show that has a child as a main character that I can think of at the moment.