I'm enjoying reading about Murphy's progress
My instinct was to put the younger
kids in a quiet place - but then I've always had dogs and never had kigs
It's pretty common and not specific to collies that the pup relates to the younger children in the family as he would his siblings - and puppies play rough, fighting, nipping games with each other. In my experience it is best that he
not think the kids are his siblings, rather that he knows he is bottom of the heap and has to do what everyone tells him. Which can be hard to achieve because of course the kids love to play and rough-house with him...
You'll have read it already, I'm sure, but now the relationship with between the children and the pup has reached that stage, read again the section at the end of the Barbara Sykes' book, "Kids, Collies and Misunderstandings." She makes it pretty clear and puts it much more eloquently than I ever could.