Hi everyone! Need some advice PLEASE! We have 2 heelers one red (12yo) and one blue(about 5mo). So the puppy was a stray and we've had her about a month. In the past few days she has started being aggressive with my 2 yo. Yesterday in the field while playing catch she was running him down, biting and even left a mark on his face. I know this is instinct for them but how can a get her to quit? By the way we have a 3yo and a 9yo and she doesn't herd them. Thanks!!

This behavior will be hard to break as the little one is so small and the dog thinks it's supposed to be herded by them. You'll just have to keep an eye on the dog and immediately correct it when it does this type of behavior. Keep the dog on a leash until the child grows taller. That's probably your best bet yet as this is basically a herder dog and it's inbred in them do this this.