Based on the friendship he had with her that he knew, and what he imagined was probably still true with her in child form, was that Nebula was very practical, very matter of fact. That was why he explained to her outright what he thought they should do. She could assess it that way, understand it, and then decide if she liked the idea or not. Even though it was quite strange that so many of the townsfolk had suddenly become children, Remus felt that they were at an advantage knowing more about them as adults that they could give good guidance as children.
He wasn't speaking down to her, regarding her as any less intelligent than he knew her to be simply because she was younger. That wasn't a tactic he thought was helpful to children at all. Just because she was a kid, didn't make her less valid. If she'd said she wanted to try something else, he would've listened and they could've figured out another way to find a book. It was just that simple.
"I suppose that could be a way to phrase it," he said when she observed aloud that he had 'powers.' He smiled a little. "I am what some would call, a 'wizard,' in that I can produce magic through spells." Remus often downplayed it; he was actually an incredibly gifted wizard in many ways, stronger and more capable to do very complicated magic including not using his wand and casting spells non-verbally. But he wasn't the type to show off; most of that was simply for his own health benefits for when he was too weak to use his wand and sometimes even speak.
"Would you like to help me flip through these books to see if we can find something about your home? There might be facts about other places and people in these pages that could interest you as well."