The sprinkler turns children into a different stimulus
Your dog is not being inconsistent; the children at the sprinkler are, to her senses, almost different creatures from the ones she ignores indoors. They move faster and for longer, in the looping, darting patterns that chase instincts are built to answer. Their voices go up into shrieks and squeals. Water flies everywhere, catching light, spraying her, soaking the grass into slippery mud. And their appearance has changed — the jeans, hoodies and shoes she has filed as 'children, normal' are replaced by bright swimwear, bare arms and legs, wet hair plastered flat. Dogs read the world in these whole pictures, and this picture is new, fast and loud, which is precisely why it gets a reaction the everyday version never does.
Chase now, judgement later: why nipping follows
Chasing in this state is rarely aggression; it is arousal with a target. A dog in full pursuit is running on movement-chase circuitry, and the classic escalation is a nip at heels, calves or a trailing hand — herding dogs aiming to turn the runner, mouthy breeds grabbing at what flashes past. That is why this game deserves management even when everyone is laughing: a nip that is nothing between dogs can frighten a child, break skin through no malice, and teach the dog that children at play are chaseable. The risk climbs as the play gets wilder, because dog and children wind each other up in a loop, and the adult watching often notices the dog's arousal last.
Management while she learns: line, fence and a job
Until she has proven steadiness, separate the roles. She can watch sprinkler play from behind a fence or gate with a chew and a person, or be on a long line held by an adult whose only job is the dog — not filming, not refereeing the water fight. Give her a defined station: a mat or bed in the shade at a distance where she can see the fun without joining it, rewarded regularly for staying. If she fixates, barks without pause or cannot take treats, she is over threshold and the kind answer is to take her inside to cool down, not to hold her closer to the action and hope.
Building sprinkler manners in small doses
Train the components on dry land first. Practise her settle on the mat while one child walks past, then jogs past at a distance, then runs — rewarding her for watching and choosing the mat. Teach the children their half of the drill: if she does get loose and chase, they stop, stand still and cross their arms, because running and squealing are fuel. Short sprinkler exposures come last: sprinkler on, dog at distance on her station, calm child movement only, ending while she is still coping. Most dogs can graduate to spectating happily; some, especially committed herders, will always be happier employed elsewhere during water games, and that is a fine outcome too.
Adults on duty, every single session
Sprinkler play is exactly the setting where supervision gets vague — several adults around, nobody specifically watching the dog. Fix that by naming the dog-watcher before the water goes on, the way you would name who watches the pool gate. Note how each session went: the distance she managed, whether she settled, what tipped her over. That record stops every sunny day starting from scratch and shows you honestly whether she is graduating or merely enduring. KinsLe's Care Teams lets you assign the dog-watching duty for each garden session and log her progress, so sprinkler days run on a rota instead of good intentions.