Niles smiled in return at Tristan's beaming smile because it just seemed the proper thing to do when a friend looked that happy. While it was true that Niles didn't know shit about babies or children, he did know a thing or two about asshole fathers. He didn't imagine that his father had ever smiled quite like Tristan was right then at the mention of his child, and that was a good thing for little miss Sophie.
"I can imagine," he said with a nod, the statement intended to blanket both statements concerning Rhiannon's tiredness and Sophie being a big part of Tristan's world.