If they want to make some aimed at the kiddies, then take those junior-age character models they made and used for a very short segment in the first and do a few kid-aimed (but still entertaining to adults) prequels with those little guys. Then leave the big boys (who could pass for their 20s anyhow) for the grown ups. There. Solved it.
(And a few adventures with kid Turtles wouldn't even have to worry about adhering to much cannon, if at all.)
Still though, with what we have, I'd rather they make it a good mix of both dark and light in their appropriate moments. Better character depth that way as well, imo, if it shows different sides to them.