We have a 9yo son. Most of his trips have been to STJ and we spent 10 days on VG in February. In no particular order, he really enjoyed -

Cow Wreck beach. He found a large conch (live) while swimming. We all enjoyed lunch.

Spring Bay and jumping off the rocks. The Baths were a "meh" for him. I think it was because it was so insanely crowded. He wanted to explore, not stand in line.

Top of the Baths for lunch and a dip in the pool.

Michael Bean show (don't know if that goes on in the summer)

Tarpoon feeding and tank talk at Saba Rock