I feel like when we took our little guy (almost 3) all of the characters were awesome. He instantly adored Minnie and Mickey and Baymax, threw himself at them but there were several like Darth Vader, who he loves, that he was shy of, but in each instance of that, it was like they instinctively knew. They interacted with big brother, or in photos he was on the end. Stuff like that. I remember one, and I can't remember who it was, just stood very still, and moved very slowly for a moment, made sure my youngest realized he was saying hello to the oldest, and all was well. Might have been Chewy. I just can't remember. But my point is, I think they see enough kids in all different phases of reactions, they will know how best to interact.
We are getting ready to go back again, and now the youngest is almost 5. He's super excited, because we've been watching youtube videos of other kids meeting the different guys, so he can see all kinds of reactions and know what ot expect.