What would you say to someone who might think of children’s picture books as “lesser” literature?

I think the first thing that I would do would be to hand them a copy of The Invention of Hugo Cabret then see if they still think that!
I would also try to explain to them that in my opinion, to be able to judge whether a work of children's literature is good or not they would have to read it keeping in mind the audience that it is written for. Children's lit is written so that children can understand it and enjoy it, which doesn't necessarily mean that it is not as high quality as a work of classic literature written for adults. To capture the minds of children, to entertain and to teach them is just as important as having a style of writing that is more difficult or complicated to understand. Getting children to read can be extremely challenging, and a great work of children's literature is one that is able to get children excited about reading...This isn't the only thing that makes children's literature good or important, but it is probably the most important thing to me.
I think I would ask that person to think about the books they read growing up, and see if the ones that they read and loved as kids were as important or exciting to them as the great works of literature they might have read as an adult... if so, then that would maybe help them to understand why children's literature is not "lesser" literature.