We just did this last week. It was hit or miss for all of my kids, usually it worked.
The really frustrating thing was on our last night my 3yo forgot his band, so I had dad and other kids go in while I went to GS to get a card for him. The cast member selected the wrong kid to reprint, but the light turned green to get in. We didn't notice until the fast pass turned blue on the 6yo's band.
So the accuracy can't even tell between a 3yo and a 6yo.