Your children are pretty young. I would recommend going with a ship-provided tour simply because they take care of all details. With young children you do not want to forget an essential you possibly did not think about.
On a ship's tour there will be guides for additional eyes. They will have the boats and know where the safe and unsafe areas may be.
Don't take chances.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.