From an outsider's perspective training/exercise with a dog looks a lot like play. Particularly curious kids won't recognize a difference.
You could just try explaining it to them in age appropriate terms.
Sorry kiddo, the dogs are training right now. It's like they're doing their homework, it helps if they're not distracted.
Or
These are very special dogs. They have lots and lots of energy because they're bred to herd sheep. If they don't get this kind of exercise they go a little crazy. Let me get them ready for playtime, ok?
In both cases if you're open to letting the kids play with the dogs, specify a time, whether that be 30 minutes or tomorrow. If you don't want them waiting around, let them know when to return.
It may be a while till they finish their homework, would you mind coming back at
time?
School age kids will be used to hearing this sort of thing from their parents and their friends' parents, so it will probably make sense to them.
