Well, they aren't saying no to Lion, so Steven is willing to take it. They allow Lion in Funland, after all, so it makes perfect sense. He reaches back, patting Lion's mane, which Lion leans into a little as his only visible reaction.

"Wow," he muses. "I've never seen different coloured lions, before. I mean... I don't think I've ever actually seen a normal lion before. Lion was kind of a big first, for me!"

First pet. First... lion. It's not a first a lot of kids have.