I would say, with kids that age, you have to ask yourself, "How will I feel if one/two/all of my children decide at the last minute that they don't want to do it?"
If you are the kind that feel, "Well it's a good experience for them no matter what, and at least they saw us do it, and they will be braver next time," then go for it. Maybe they will do it no problem.
But if you are the kind that feels, "What a waste! All that time and money and they didn't even get in the water!," then don't do it. There is a decent chance they won't be willing to do it.
We did a dolphin encounter (on Grand Cayman) and the family next to us had a boy, maybe 6 years old. At first, he wanted to do it, but then when it came time to actually get in the water, he wouldn't get in. His parents gently tried to persuade him, and the staff encouraged him, but when he still wouldn't get in, they let him stay on the edge and watch. Then, when he saw everyone else do it, he jumped in and got his turn. But it could just as easily gone the other way.