God help me if my nephew slapped Tatiana.
First of all, I would scold the child.
Second of all, I would talk to the parents, and explain why I would put some boundaries between their children and my dog. I consider myself Tatiana's protector. I take that role seriously.
Incidentally, if the situation was reversed, and Tatiana bitten one of their kids, I would completely understand if they behaved the way I would. Thus, I think I'm acting with integrity.