Parking lots and fields are no place to introduce a child to karting. Even seemingly well organized events can be dangerous when an inexperienced driver is involved. This link leads to a tragic story that serves as a reminder of how quickly things can go wrong.
I’m all for introducing kids to karting but it should be done with properly designed and prepared equipment that is suitable for the age and experience of the driver and the driving should be confined to properly designed and maintained dedicated racing facilities.
The chassis you have there is designed to fit an adult laying down and will sit about 1″ off the ground if fitted with typical kart tires and wheels. The kart is intended to be used on large road racing circuits and will not turn sharp due to the long wheel base. It would be worse than starting from scratch to convert it to a machine that suits a 6 year old driver.
Hope you take the good advice offered here by several well intentioned posters and re think your plans.