Was it Jeph Loeb or Paul Dini who speculated that Batman took in Dick Grayson because he anticipated his terrifying potential, and realised that his anger had to be chanelled for good? I think it was one or the other, and it's a fine idea.
And, of course, there's the Frank Miller approach, where Robin is a barely-contained psychopath who eventually becomes a serial killer Joker wannabe with a healing factor.
God, I hate DK2. I wish I could hate it
to death.
Anyways, I would prefer to avoid tampering too much with his background. A lot of the proposals for including Robin have been extremely revisionary (many based on Dick, before meeting Bruce, going out and doing the vigilante thing on his own). Obviously this is an attempt to work around some of the more unbelievable aspects of Robin, but I think we can do this in a less revisionary way.
Yes, being a circus acrobat isn't going to prepare him for combat, but then, we don't necessarily need him to prepared for combat. I don't think there are many ways to make this work in the atmosphere of these films, but I think the most obvious way is Bruce to use combat training (among other things) as a method of channelling Dick's anger, whereas Bruce, at that age, had nothing and end up lost. I think it's important that the idea of a sidekick never occur to Bruce. It would just be a opportunity to vent and focus on something useful. So, that's how we start to prepare Dick for combat.
After Dick (who let's say is fifteen or sixteen) finds out Bruce is Batman, obviously Dick has ideas. Dick goes out on his own to try and put what he's learned to use. Of course, this is not acceptable and Batman wants to put a stop to it. When conventional solutions don't work, Bruce decides the solution may be to offer Dick the closure Bruce didn't have: bring in Zucco. I think the idea here would be to involve Dick in the investigation, so that when Zucco goes down, Dick feels like he's accomplished something and can let it go. Leading up to this, we'd see a Dick's training. I think that while combat is essential, it would be useful to focus on investigative technique, surveillance techniques. We perhaps see Dick doing stuff like Bruce's hobo-disguise eaves-dropping in Begins; utterly dangerless tasks that involve him in the effort to get Zucco. Not only are these useful in nailing Zucco without putting Dick in any danger, but they also create a precedent for Robin to be focused on support, not combat. I'm sort of torn as to whether or not Batman would actually involve him in the physical
capture of Zucco, as he did in DV. Bruce might, or, if not, Dick would get involved against Bruce's will. If Bruce
does allow it, that solves the costume problem: even though it's supposed to be a one-time deal, Dick has to be protected, so Bruce might put something together for him. Perhaps he cannibalizes his old suits to build one for Dick and slaps a flack jacket on top (for the tunic). Obviously it wouldn't be coloured red yet, nor would it have the R on it, and maybe not even a cape. It would be purely functional.
Anyways, it's tricky. If Bruce allows Dick to participate in the physical takedown, it has to be designed so that Bruce has solid reason to believe Dick will be safe. If he doesn't, Dick will have to find another way to get a costume, and more importantly, Bruce's plan to give Dick closure is a little weaker. Either way, when Zucco is down, it's over as far as bruce is concerned.
Naturally, the plan backfires, and Dick isn't satisfied to put it away and go back to normalcy. We're near the end of the film now, so the next part is easy: Dick, having reworked the armour into a Robin costume, gets involved in the climax without Bruce's consent. They work together because there's no option at this point, with Dick already involved.
When all is said and done, it might be better to leave Dick's future as Robin uncertain. I don't know that it would work for bruce to say "You helped me beat the Riddler, welcome aboard!" Bruce might realize that he's not going to be able to dissuade Dick, so maybe Bruce simply allows Dick the opportunity to prove himself. Not immediately in the field, but through whatever tests or methods Batman might have ahead; a long, gruelling process where, knowing Dick is going to get out on his own eventually, Bruce does his best to build a better version of himself in Dick. He can't stop him at this point, so he opts to work to keep him alive.
Alternative, it could simply be ambiguous: you have bruce grimly contemplating the matter at the end of the film; understanding the insanity of it, but unsure of a way to really dissuade Dick. Some of the audience go home assuming that this is the end of Robin. others, the opposite.
Bottom line, it's never going to be cut and dry, where you can say "Batman definitely did the right thing." It
shouldn't be. Conflict is what fiction is about. Hard decisions, sometimes insane ones. It's a challenge to make this one workable, but I don't think it's insurmountable.
Ah, one thing I forgot: I think it's important to establish Dick as an extremely gifted child. Not just physically, but mentally as well. Like the Battle School students in Ender's Game (though not to that degree, as those damn kids were smarter than the adults, who were themselves smart).