My two went when DD was 4 and DS was 7. They had an absolute blast, and somewhere or other I have great pics of them, all decked out in their bandanas and lifejackets. I'll admit I was a little nervous, but you could already tell on the dock that they would be very well looked after.
They went several years ago, and at that time the snack they served was a PB sandwich and grapes, if I recall. To me, the greatest risk would be for any child was who was allergic to peanuts! I found it incredible that Disney would take any chances and offer a peanut-containing snack to children, especially when their parents weren't present. With any luck, Disney has changed the menu, and that risk will have been removed.
Cheers,