On the masks and related snorkeling equipment, renting would seem to make the most sense. This site, which has facilities in both Tortola and JVD (https://www.bviscubaco.com/find_us.shtml) advertises that it rents all manner of diving equipment, including a full set of fins, masks and snorkels for $6, and I'd be surprised if it did not have equipment for children:

EQUIPMENT RENTAL: lowest prices in the BVI
FREE DELIVERY & DIVE SITE BRIEFING

BCD's & Regulators (SHERWOOD) . . . . . . . . . . . . .$9 each
Scuba Tanks, Tank Racks, & Air fills . . . . . . . . . . . . . . . .$5
Masks, Fins, Snorkels (full set) . . . . . . . . . . . . . . . . . . . .$6
Wetsuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .$5
Weights & Belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .$3

https://www.bviscubaco.com/scuba_rates.shtml