Can someone explain how Dick Grayson aged from a kid to an adult while this Batman had 4 sidekicks? That would mean Batman would have had to be Batman for at least 15 years in this universe.
If you're referring to the New 52, it's been shown that of the 6 years Batman has been active, Dick (at 16 years old) became part of the team 1 year in, and after a couple months of training, officially became Robin as he first put on the suit to assist Batman against Lady Shiva (she beat both of them easily, although she decided to spare them because she saw the potential in Dick).
This is going by what Kyle Higgins has created as the New 52 origins of Nightwing, but since he never got to tell that story, it's still not the official origin. Still, I feel like it should be mentioned as it helps the whole '4 Robins in 5 years' thing: Dick eventually left Batman and dropped the Robin identity when he had a lead on Tony Zucco (who had returned to Gotham from hiding to try to speak to his daughter, Sonia). Bruce felt Dick could end up going over the edge, which as we know, Dick Grayson of all people would never do. He chose the identity of Nightwing (Higgins doesn't really mention any connections to Superman here...) to hunt down Zucco and being him to justice.
Of course, this doesn't pan out, and so Dick is now Nightwing and Bruce is Robin-less. At this point, I imagine, would be when Jason would've entered the mold, and after just starting his time as Robin, would've died. After some time, Tim showed up as Red Robin, and during his tenure as Batman's sidekick, Damian showed up, eventually became Robin, kicked Tim out, and would've teamed up with Dick (as Dick was Batman for a time in the New 52 as well; what happens to Bruce isn't really specified, although it was said he had died in this universe as well).
So essentially, it went like this:
-Richard Grayson (1st Robin: started as Robin at 16 years old and 1 year into the 6 year career of Batman, soon after left and became Nightwing, assumed the the mantle of Batman during the 5th year of the 6 years Batman has been active, and then returned to being Nightwing at 21 years old and 6 years into Batman's career)
-Jason Todd (2nd Robin: started out at a comparable age to Dick after Dick had left to become Nightwing, died soon after, and later returned alive as the Red Hood)
-Tim Drake (Red Robin: started out at an age around 15-16 years old some time into Batman's career and some time after Jason's death, left the team of the Dynamic Duo after Damian had shown up, and later formed the Teen Titans 6 years into Batman's career)
-Damian Wayne (4th Robin: started at an age close to 10 years old during Tim's tenure as Bruce's partner and just before his Bruce's death, soon became the Robin to Dick's Batman around the 5th year of the 6 years that Batman has been active, served as Robin to his father after he had returned during the 6th of the 6 years, and died during that same year)
Anyways, as for this movie, it hasn't been stated how many years Batman and Nightwing have been active and how old they were when they started out, so 15 years may be plausible, as is the assumption that Dick started out when he was 8-12 years old, which is common in almost every other story.