Why the size change changes the game
A young dog who once bounced harmlessly against a toddler can now knock that same child flat with one happy turn, even though her intentions have not changed at all. She has gained weight and momentum faster than she has gained body awareness, and most adolescent dogs still misjudge how hard they land when they spin, leap, or shoulder into a game. Your youngest is also at face height for mouthing and pawing, which makes an overexcited greeting riskier than it would be for an older child who can step back and stay upright.
That does not mean play has to stop. It means the style of play has to grow up alongside her, shifting from free-for-all romps to structured games where an adult sets the pace and the dog stays calmer than the children do.
Games that keep four feet on the floor
Choose games where excitement travels through a toy or a task rather than through bodies. Fetch down a hallway with a soft toy works well because she runs away from the children to chase and returns to an adult hand for the swap. A find-it game, where the children hide a toy while an adult holds her, then release her to search, gives the youngest a starring role without any physical contact at all.
Tug can stay on the list only with strict handling: an adult holds one end with the child, the toy stays below waist height, and the game stops the instant teeth touch skin or she starts leaping for it. Drop chasing games, tag, and any wrestling on the floor entirely for now, because those are the games that teach a heavy young dog that bowling into small people is how fun starts.
What the children need to learn
Give each child two rehearsed moves and practise them when the dog is calm, not mid-game. The first is statue: feet planted, arms folded, face turned slightly away, staying still until an adult steps in, which removes the squealing and flapping that escalate a bouncy dog. The second is a clear swap: if she grabs a sleeve or a toy they are holding, they let go and call an adult instead of pulling back and turning it into tug.
Seat your youngest higher than floor level during play, on a sturdy chair or the edge of a sofa with feet tucked up, so a shoulder-bump passes below them rather than through them. Never leave even a trusted young dog alone with the children while you answer the door or make tea, because the knock-down incidents almost always happen in those unsupervised ninety seconds.
Ending sessions before they boil over
Keep sessions short, around five to ten minutes, and finish while she is still responding to her name and taking treats gently. Signs she is tipping over the edge include hard staring at a running child, grabbing at clothes, barking in faces, and ignoring a cue she knows well. At that point play pauses without drama: an adult leads her away for a drink, a chew in her own space, and a reset.
Rotate who rests and who plays if you have several children, so the dog is never managing three excited small people at once, and keep a baby gate or closed door as the separation tool rather than expecting the youngest to enforce a boundary. Over a few weeks of this pattern most young dogs learn that children predict calm, seated, treat-and-toy games, which is exactly the association you want carrying into her adult size. Selected family members can receive profile access from her owner via KinsLe Family Sharing.