I would say Danny actually has a bit of a mini arc in the film. I mean he?s not a hugely important character, but he is in a good chunk of the film.
We first see him when April?s boss stops over at her place and Danny is with. He?s a little punk and we learn shortly after that he?s hanging out with the Foot. He gives away that he?s seen the Turtles at April?s place and then inadvertently leads the Foot to them.
Later on he feels wrong about the way Splinter is being held captive and tells the Turtles which leads them to the Foot?s lair so Splinter can be saved and Shredder and the Foot can be dealt with.
He actually does a fair amount for just being a side punk kid role.