Why children are the ones he chooses to round up
From a herding dog's point of view, children are the obvious job: they move fast, change direction suddenly, shriek at the exciting parts and scatter, then bunch up again. Circling to gather them, barking to move them along and nipping at heels or trailing sleeves are the tools his instincts supply for stock that will not stay together. He is not being dominant or naughty, and the children have usually done nothing wrong either; they are simply playing exactly the game his breeding was designed to control. The risk, though, is real: a nip aimed at a heel can land on a small leg, and a knocked-over toddler frightens everyone. So this behaviour gets managed seriously from the first day, not laughed off as clever.
Split the garden into two different games
While training runs, running games and the dog do not share a space. Children charge around the garden or the park while your dog is indoors, on a lead with an adult or settled behind a gate with a chew. In the dog's company, the children's games become the slower kind: den building, ball rolling along the ground, reading on a blanket. This is not a permanent exile for anyone; it is removing the trigger combination, speed plus scattering plus squealing, until his calm-watching skills are strong enough to be tested carefully. Every adult who supervises, including grandparents and babysitters, needs the same simple rule, because one afternoon of tag undoes a week of careful setup.
Train the watchers on both sides of the game
Teach the children one clear response: if the dog starts circling or fixating, they freeze, stand tall like trees with arms folded, and look at an adult. Running and squealing feed the sequence, while stillness bores it. On the dog's side, build a mat settle within sight of family life and reward him heavily for lying there while ordinary movement happens around him, starting with adults walking, then a child walking, then short bursts of livelier play at a distance he can handle. Practise his recall away from movement in the garden with no children present until it is snappy, then test it at the edge of calm family activity, on a lead, with food he cannot resist.
Supervision standards that do not relax
An adult's job during this training is watching, not watching while cooking, phoning or gardening with their back turned; if no adult can actively supervise, the dog and the running children are in separate spaces, full stop. Teach children never to run from him, pull his collar or hug him when he is aroused, and give the dog a retreat spot children are not allowed to enter. If he has bitten, even lightly, or he cannot settle at any distance from playing children, stop home training and get a qualified behaviour professional to assess him in person, alongside a vet check for pain that could lower his tolerance. The rules only protect everyone if every caregiver actually knows them, so talk the freeze rule and his settle routine through with the adults who supervise him rather than assuming they have picked them up. Brief grandparents before the next garden game, then invite the family members you pick to view his profile through KinsLe Family Sharing, and nobody beyond those names gets in.