I've been in the Hole a number of times and have seen sharks on every trip.
My first experience started right after I dropped into the water off of the boat. I was still at the surface, looked down and there was a big one lazily passing beneath me, no more than 15-20' down. It paid me no mind at all.
One of my more recent experiences was quite a bit more exciting. We had just begun our ascent from the stalactites and I spied a beauty out in the water column. I was shining my flashlight up and down the length of him. It turned towards me and began to swim my way. I flicked off my light and started to swim away with the group, but my spidey senses were tingling. I turned, and there it was coming up behind me. I went vertical in the water and watched it approach, with my heart thudding in my chest. It passed ever so slooowly, no more than a foot beneath my fin tips, first its snout, then its eyes, then its dorsal. Then . . .
it was gone. In relief and wonder I turned back to the group, most of whom had been watching the scene unfold as well. As if nothing remarkable had occurred, we carried on with our gradual ascent up the wall.
After the safety stop and upon surfacing, I yelled out "Again, Again!! I want to do it again!!" (BIG GRIN)
My dive buddy told me that he was very jealous and that he wished it had happened to him. Ruefully, I was thinking that I wished it had happened to him too. LOL.
In any event, don't be afraid and even if you don't make it to the Blue Hole, you'll more than likely still be able to tell the kids that you saw sharks. There's plenty of nurse sharks around and they are very cool. They look scary but they aren't. Quite beautiful, quite beautiful.