Whiting is fine. In general the bigger deep sea fish that are higher up the food chain have higher mercury levels.
Yeah that!

Big fish eat little fish so the big fish have higher mercury content.

Flake once every 2 weeks is fine or you can have ither fish more regularly. (whiting, butter fish, blue grenidear) Kids shouldnt eat flake either.

Tinned tuna is not a problem at all

Enjoy