... I'm a big fan of Robin (especially Tim Drake), but if they're gonna use Robin in the movie, I feel it really does have to be a very young teenage actor portraying him, and not a 20-something year old dude...

In the comics, Robin is a KID. He needs to be portrayed as such, especially if we're talking about him first getting 'adopted' by the Batman.

I also hate that about the Spider-Man movies btw. Peter Parker is supposed to be a skinny small 15 year old or something when he gets bitten by the spider and becomes Spider-Man, yet they keep casting full-grown actors who are already in their twenties...(!)

It's just stupid.

... that somehow having a kid in such a movie would mean the movie and the world it depicts would have to be less violent somehow...

Personally, I've never understood that logic of thinking. In real life, kids are being placed in mature situations all the time. Parents die by illnesses, accidents or violence, kids are caught up in wars or gang-related incidents, etc etc. And they cope with it. They have to. That's drama, but also reality.

As far as I'm concerned, using an actual kid in the role of Robin would make the violence, and the setting of that violence in that created world, all the more powerful. And Robin, as a kid, could very well deal with all that.

Will parentgroups complain? Probably. So what. Let them.