Start in the pool for basic skills and comfort level. Many things can interfere with this at a snorkeling site, even somewhere as perfect as Blue Heron Bridge. Visibility, salt water in eyes/mouth, marine life, just to name a few, can be a major distraction while learning the basics. I'm not for spending money unnecessarily, but maybe the place you are staying offers a snorkeling class? The skills are not a tough thing, but maybe even YOU could learn some techniques that will help you when out there. Once they are ready, only take one at a time or have your wife with one and you with the other. My kids are recently OW certified, but have been snorkeling in OW since 4 and 5 years old. It can be a challenge to pay attention to more than one at a time, and even harder to assist one while maintaining supervision of the others, particularly at this age. Snorkel vests are a plus, but one more piece of equipment to deal with learning how to use, and being comfortable using. I like the idea of a float/flag. If you are in a designated swimming/snorkeling area, they are not mandatory, but give the kids (and you) some peace of mind for safety. Enjoy your time in the area. We have a lot to offer!