My 7 year old wants to know why shrimp and lobsters have to be in water to survive, but not crabs?

First of all, crabs as a whole cannot survive on land. Only a relatively few species can leave the ocean.

So, why have some crabs made it out of the water while lobsters and shrimp have not? Body structure.

Shrimp are primarily swimming creatures. Their legs are not remotely designed to support their weight. They are not likely to evolve into terrestrial animals from their current condition.

Lobsters may live on the submarine ground, but they have those long tails. Those tails would not be easily supported above the water, leaving them to drag in the dirt. Since the lobster has rather important stuff under it's tail (including gills), this would be fatal. They aren't going to be able to make the transition to land animal easily.

Crabs were all set to evolve to life on the land just from how they evolved under water. They lack the long dragging tail, their gills are better protected, they have strong body lifting legs. All that's really needed is some futher modification to the gills to let them survive short jaunts on land, things easy to evolve while living near the tide lines.

There are even some crabs in the amazon that are more terrestrial than marine!

you purely picked 3 ideal possibilities! i will bypass with fish for as we communicate, a walleye possibly? final week i grow to be in Mississippi and had Crab and Shrimp galore. Love lobster yet i want walleye ideal now.

Crabs are like frogs, they are amphibians, they can live in and outside the water. Just tell your son/daughter that this is a special class of animals.

Crab may store salt water in their bodies... while shrimp, prawns, and lobsters can't. Just my assumption because if you rear a crab in fresh water, it'd probably die off. ^^

That's a good question he asked! I'd like to see the answer to this.