We go camoing all of the time with a 40' toy-hauler that has a hitch that fits on a goose neck hitch. We also have 4 small children. Is there any type of shell that can bee added to my crew cab that would give the children extra space but not interfer with the hitch?

You'll probably have problems with the front of the trailer hitting the shell when turning if you try that. If there is something available, it would be so small that you wouldn't be able to give them much room, also, most states have seatbelt laws, especially for kids so you might get a ticket if they're in the bed and not strapped in.