thats an interesting idea, i myself have lots of kiddie pool ideas rolling around in my head, what are you planning on making it with cement, hypertufa, i think its possible. you probably would have to play around with some mixes to see which ones would work better. i thought of getting three kiddie pools of different sizes and making them spill into one another .

My mother did not use a kiddy pool but built a 6' pool approx 4" deep using metal lathe as the support.

It weights 400 pounds. The rought iron base she had to support it - well, won't.

Certainly consider building on site, maybe a way to cut away the pool after the cement cures....

My dad was experimenting with an idea a few years back. He used the blue styrofoam insulation you get at Lowe's or wherever in a 4X8 sheet. It is easy to work with to get your shape. build the shape with the blueboard, cover it with chicken wire, then smooth on your concrete mixture. The chicken wire will hold it, may need several coats. I remember he used a mix higher in Portland than some, and mix it as dry as you can. This makes it stronger.