First, let's start with:
Dick Grayson/Nightwing: Thomas McDonnell or Jared Padalecki-
If you watch "The 100", you'll see he is the perfect leader, fighter, and he's built. With that long hair of his, he will be a very good Nightwing.
If Jared was to shave his beard and keep his long hair, he would play an incredible Nightwing. He's already a pro at taking down supernatural monsters, why can't he be a pro at taking down villians, too?
Jason Todd/Robin/Red Hood: Jensen Ackles-
There is no other person to play Red Hood except Jensen Ackles. He is an amazing actor. He's serious, built, and he will look really awesome fighting Batman.
Tim Drake/Robin/Red Robin:Logan Lerman-
If anyone can play Tim Drake, it's Logan Lerman. He played a really good Perseus, in the Percy Jackson series. He's built and knows he to be a leader.
Damien Wayne/Robin: Chandler Riggs-
If you watch The walking dead, you'll know Carl has a smart mouth and knows how to handle himself. If you know how Damien Wayne is in the comics, he is the same. Chandler Riggs will make a great Damien Wayne.
Cassie Kelley/Robin: Jena Malone
I mean come on, who else can play Cassie Kelley. If she keeps her red hair and puts on that Robin outfit, she will look just like Cassie. If you watch The hunger games series, you can tell she already kicks ass. Might as well kick ass as Robin.