The guys shooting the shell will be the ones to make the steps and seats in the pool and spa. To make different heights in the spa they will shoot the gunite in the area you want the high spot how ever high you want that area to be and finish it. This will serve you well in the future for the kids and other people.
Depth+kids=they will only be small for a couple of yrs until they are big enough to tip toe then walk in the shallow end. I always say to begin with the end in mind so................that is the reason for saying 4' to 6'. While they are small they will use the spa as a small pool as well as the shelf will be there play area.
Seems as if you have the lights all set up. NICE! Don't forget to have them install one over the equipment pad. You will thank me the first time you have to go out there at night!
Kim