I just took my almost 6 yr old. He had a little trouble with the mask and snorkel. Also the vests they give you are just neoprene, not flotation devices, you can ask for a PFD for the young one, as you are not allowed to kick your feet and many young ones cannot stay afloat otherwise.
There is no plexiglass, the fish are free to come up to the surface, but they just choose not to.