For the record, I'd say the juxtaposition between the Batman's nostalgic reminiscences of the lake and the present-day reality of it are actually quite effective - especially considering who it's coming from. Bruce's happy childhood memories are very important to him, given how abruptly his childhood came to a screeching halt, and seeing a part of his childhood despoiled like that would likely be quite upsetting to him, much more so than with some other characters. Remember - as has been said many times before, Batman likes kids. He's good with kids. Part of his whole reason for doing what he does is so no other kid will have to go through what he went through. Therefore, the opinions of kids really matter to him, and so does making sure that they enjoy themselves and make the most of their childhood while they can. I can see him genuinely looking forward to taking a bunch of slum kids on an outing, and taking them to a place that he remembers fondly. Having that outing ruined by pollution and litter is not merely embarrassing, it's upsetting, and a good motivation for him to make sure that things won't get worse if he can help it. He wants a better world for these kids, and perhaps more importantly, he wants them to be able to look forward to a better world. Really, if you wanted to pound home a message about keeping the world clean for the next generation, you couldn't pick a better spokesman than Batman.