On the one hand, I agree with garbage and recycling being private property... "Dumpster diving" is one of the better known ways of retrieving sensitive information that can be used for identity theft. On the other hand, depending on the age of the kids, they probably aren't out to steal anyone's identity (esp. in the recycle bin) but are rather being enterprising by grabbing aluminum out of the bins and selling it for a little extra cash (it still is getting recycled, and if the property owner had wanted that cash, they wouldn't have thrown it in the recycle bin in the first place). Or perhaps they have a school project and were trying to find a bunch of empty milk jugs or 2 liter bottles. As long as they weren't hurting anyone or making a mess on the street, I wouldn't worry about it.
I know when I was young and used to live in NY where there was a 5-cent refund on pop / beer cans and bottles, I used to carry along with me some garbage bags whenever the family would go someplace where there might be a lot of empties lying around. For example, I'd go through the garbage bins lining the street of a town when they were celebrating their Maple Festival, and I could end up making and extra $5 for my endeavors. Technically, I was probably breaking some law by doing that, but realistically, I was helping the recycling effort and was getting paid as well.