Small kids are very interesting to a lot of dogs - they look and move differently than adults, and often move about at higher speeds. Border collies in particular are highly motion-sensitive, so it's not surprising your dog's taken notice of them! That and pups can go through developmental stages where certain things are more fascinating or scary for a little while. I'd start by asking him to refocus on you the moment he spots kids, and reward with an awesome treat (a high-value food treat or a game with a favorite toy, depending on what he likes). If he's too fixated on the kids to get his attention, take a step back and teach him a really solid "watch" or "focus" cue at home, making sure he's well rewarded for turning towards you.
Your goal is to teach him that it's more rewarding and interesting to pay attention to you, and that children aren't all that interesting, after all.
EDIT: also wanted to add that if you can't get him to focus on you with children around, don't keep trying. You don't want him to practice getting overstimulated and amped up, because that usually leads to it happening faster next time. Either increase the distance between him and the child, or leave the area completely and try again when you've got a bit better control of the situation.
Your goal is to teach him that it's more rewarding and interesting to pay attention to you, and that children aren't all that interesting, after all.
EDIT: also wanted to add that if you can't get him to focus on you with children around, don't keep trying. You don't want him to practice getting overstimulated and amped up, because that usually leads to it happening faster next time. Either increase the distance between him and the child, or leave the area completely and try again when you've got a bit better control of the situation.
