It is a pilot fish. I wonder if you checked it for your child. My son saw it in the small column on the bottom of the sea.

Baidu Encyclopedia said: Pilot fish often swim in groups in front of sharks, so that they can not only "fake the power of the tiger", but also have "leftovers" to eat. A small pilot fish may even swim into the shark's mouth to nibble on food between its teeth. They will also swim in front of the boat, so they are called "pilot fish". Sharks rarely eat this lure fish.