My adult daughter, my husband and I went did the helmet dive at Grand Turk and we loved it. I was probably the most apprehensive because I can be a little claustrophobic but it didn't bother me at all. I really wanted to do this because the daughter we were with actually works with these type of helmets in a deep dive program at SeaWorld. She actually puts people in a cage in the shark tank there in these type of helmets.
Your daughter may not be able to participate though. On the carnival site the info for the helmet dive says children must be 8 years old.
It was a lot of fun and the people running it were very good at their job. They made everyone feel at ease and were very safety conscious. The fish we saw were beautiful and it was a lot of fun walking on the bottom ( it felt like it must feel on the moon - you just bob along ).
Hope this helps. If you have any more questions, feel free to ask.