
This is typical for all puppies, but Border Collies more so. They have been selected to have the instinct to do this, it is herding!
Your pup will not stop doing this of it's own accord as it finds the behaviour rewarding, as well as it being instinctive.
You will need to train him and the children. You must not allow the kids to run around inhis presence. If they are playing, you should call him to sit by you, so that in his mind playing/running children equates with go to Mum for a quiet cuddle and a treat. Distractipon and redirecting his behaviour is the only thing that will work.
I would suggest partioning the garden, so that the kids go in the top of the garden to play, where he isn't allowed, and you do some treaining or play fetch a ball type games with him.
As he matures he will realise that he isn't supposed to herd the children, but the won't know this unless you teach him.