I'd be reluctant to put a 4-year old on compressed air, even that shallow, yet. Only because I'm not entirely sure they'd be old enough to understand they can never hold that breath if they rotate or surface. Our instructor (who may have been overly conservative, or simply very careful) emphasized that an adult diver (not exhaling) simply rotating from horizontal to vertical position, unless they rotated "about" their lungs, could generate enough pressure change to blow out an adult lung, from a 4-6' depth change.
The whistle is a good idea but I'd file it under "more stuff". That is, nice if it doesn't fall off & away as they submerge. Or they don't panic and forget about it, etc. And as long as they don't toot on it for fun, so people ignore it. People also routinely suggest whistles for open water scuba, and I've got to tell you, unless you are downwind from the whistler? No one ever hears it. But it's a nice way to pacify the guy blowing it.
I like the big hairy dog, despite the ah, collateral damage it brings. Even if all they do is bark, the sound of an upset barking dog often brings help.