This is very, very important. DO NOT do anything about this on your own. Do not try to confront the person, do not try to expose the person, do not try to lure them into a trap and record it all on video, etc. The ONLY thing you should do is to report it to the police. If you attempt to take matters into your own hands, very bad things can happen. Best case scenario, you risk ruining the life of a perfectly innocent person by making the world think they are a pedophile when they are not. Worst case scenario, you allow someone who is a pedophile and is targeting kids to get away with it.
The reason for this is because of the way the legal system works. If you attempt to take matters into your own hands, any "evidence" you collect will not and can not be used in court. Additionally, it will call into question EVERYTHING you have previously provided to the police, so they will no longer be able to use that to make a case against this person. If his alleged actions become common knowledge, finding a non-biased jury becomes next to impossible, and he gets off free because of a miss-trial.
It also may be that the police are using this guy to get others as well. If this person were to think someone were on to him, it may stop him from doing anything right here right now, but it might also mean that dozens of other individuals, potentially all over the world, will get away because the police were not able to get to them via him. It could just be that they don't have enough evidence to make a case against this person, and they need more time to collect more information.
It may seem brutal to allow more kids to fall into this trap, but the alternative would, in the end, be far worse for a lot more people. You wouldn't really even be helping the kids in the moment because again, if all the evidence is tainted by a vigilante, they have no viable case against him.
The best thing you can do is to not interfere at all. If you know any people in the area, family friends, etc, you might caution them about this, but don't make a big deal of it, and ensure that they don't either.
The police know what they are doing. This is what they do for a living. They know how to use the legal system to their advantage, trying to take matters into your own hands will just slow them down.