I have never heard of electric eels in Qatari waters.
Lion (chicken) fish if you go further out.
February to April sea snakes breed and will attack anything. Generally speaking, if you present them with a fist they have such a small haed that they cannot find anywhere to bite. Give them an open handed push and they will have you between the fingers.
Our kids almost grew up on the beach it's as safe as houses and I'm sure your grandkid will enjoy it.
Scorpions......we only saw two in 17 years (apart from at the zoo).
Roaches can be a problem as they are quick (known as Bombay Runners)and the flies bite at times