Generally speaking, you need to be in the wizard's eyeline. If he's standing on the stairs, and the person letting you inside tells you to head directly to the wall in front of the stairs, stand there. If he's at the counter, stand in front of him towards the left next to the steps. But of course, that's if you are the first ones in in your group. If not, then someone else will have grabbed those spots and they aren't inclined to reward pushiness (physically speaking).
Of course, there really is no way to guarantee anything. They don't go by age, they don't go by who's the most energetic or excited, no gender is more likely. But know that if your kid isn't playing along, they'll turn to someone else who will play along.
My recommendation, tell your kid it's not going to happen, and then let them have the more magical experience if it does.
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