Usopp hated to be called a mother, he couldn't imagine a more feminine thing to be called, and that went against all he was striving to be. But everyone, despite the name-calling, loved how good Usopp was with taking care of everyone. They had Chopper for any sort of physical injuries; Usopp was more of a precautionary person: blankets, sweaters, and other such motherly advice.
Sanji thought it was hilarious that Usopp be called a mother. But when Usopp found him taking care of a child, all joking stopped. Sanji had a gift with kids, he had a brotherly way of talking to them, of giving them back the hope they'd lost, and Usopp was the only one to witness all of these things Sanji did for kids.
On an island slightly off course, everyone but Zoro had left for land, for food, or grocery shopping, or whatever they felt they needed to do. Usopp was scouting ideas for weapons when he saw Sanji out of the corner of his eye. His first instinct was to call out but then he noticed someone with the chef: a little girl.
Usopp was shocked, he would have guessed that Sanji left to get food but found women instead, not a kid.
The girl was crying and Sanji, knelt by her side, seemed to be helping slightly, with whatever he was saying to her. When she had finally stopped, he stood up, her hand in his, and the two left. Completely curious, Usopp followed the two to find Sanji talking to an older woman, the girl crying with happiness. He was close enough to hear, "Take care, Miss." Sanji turned to see Usopp, blatantly standing in the street.
"Oh. Usopp. What? Are you done mother-shopping?"
Usopp frowned, but quickly turned it around, "What was that about Sanji? I thought you liked older women." Usopp snickered, but Sanji didn't find the joke nearly as funny.
"Oh ha. Ha." He pulled Usopp close by the collar and hissed, "I was helping her find her mother. Any gentleman would have done the same." Usopp was a little taken aback, feeling slightly guilty over the comment. He pushed Sanji's hands off his shirt and smiled.
"Sorry, Sanji. I guess you just don't seem like the kind of guy to have a way with children."
Sanji shrugged, his irritated expression melting into a smile that matched Usopp's.
"Well we can't all be mothers." He said, eyeing Usopp, "Some of us just have a way with kids." Usopp caught the insult in the beginning, but grinned.
"I guess that's why we have fathers."