The recipe I've been using has been -
1.5 cups corn flour
2.5 cups water
A few drops of food colouring
Mix the corn flour and water together. Then heat over a gentle heat until it thickens. You want thick custard consistency.
This amount makes about 2 jam jars full. I just store it in the fridge and then pour out a little and add a few drops of food colouring. It keeps well for about a week or so - well that's how long I've had it before it was used up. It does separate a little by the end but a good stir (and a little heat if necessary) gets it back to the consistency you want.
The kids love it and when the littlest one inevitably decides to see what it tastes like there's no worries about anything nasty.