I’ve seen it every day for the last 43 years.
With 5 or 6 going all at once, sometimes the back yard looks like a huge pinball machine..:)
Maybe by the next time you visit, the dog will have emotionally matured *or* the niece will have properly trained it.
I know some of the things dogs do are unnerving to us but they tend to understand our body language better than we understand theirs.
Bottom line is, if you feel uncomfortable having your child near the dog, then don’t.
It’s your child, your choice and your right.
lol....it’s probably pretty funny to watch dogs do it to one another. And if you are around the dog a lot I suppose it wouldn’t be so unnerving. I have a rat terrier, or should I say terror...lol She’s actually pretty well behaved, took a lot of work to get here there. I can see that terriers need a strong personality and a large terrier could really be trouble if not well controlled.