There are places that make cages for flat Karts that you bolt on and add belts, you take rear bumper off your chassis and rear of cage slides over chassis tubing then the front is built like a seat strut and bolts on chassis tubing they work very well, BUT do your homework not all tracks allow them most run a true champ kart which is built specific with cage being welded to the frame, I know J.C. Specialty makes bolt on cages you can find there web site top of home page, but you should be able to find a shop close to you to avoid shipping.
Heads up MOST kids that learn in a flat kart find out they don't like the cage after they try it, especially the true champ buggies.