I'd imagine they could pop in a meet and greet later on if they wanted to, depending on the exact layout of the land. I'm curious how much they promote this whole thing, since it'll pretty much be new by itself (as opposed to being part of a bigger season of new offerings). I can't imagine anyone planning a trip just for it unless they're a huge Toy Story fan AND have no knowledge of what's actually there.
I will say that kids who's first experience with the park is after this opens will probably like the land a lot more than those of us who've been before. To them it'll be a land with a mini-roller coaster, a spinner, and Toy Story Mania, whereas to the rest of us its a spinner and a mini-roller coaster... if that makes sense.