Open the can of salmon and drain any excess liquid. Flake the salmon into small pieces, checking carefully to ensure there are no bones.
Cut the avocado in half, remove the pit, and scoop out the flesh. Mash it in the bowl with the fish until smooth or leave it slightly chunky, depending on your child's preference.
Dice or mince the watermelon into small pieces.
Mince the cilantro and mint and add to bowl. Squeeze lime juice into the bowl and stir to combine.
For younger babies, you can put the whole completed mixture into a blender or food processor to blend into a smooth consistency.