Visual standard for gas flame levels
An issue you'll have to deal with is diameter. A high flame on a smaller ring may put out more or less heat than a low flame on a bigger ring. While you might think the pan size would cancel that out, a big pan may need to go on a small ring for a gentle simmer. The height between the burner and the pan rest isn't exactly standardised either. 

You might do better to think in terms of the pictures on the dial: big flame, little flame, and halfway in between. Even then you'd have to normalise for pan size. Perhaps assume a pan a bit wider than the biggest flame that ring delivers. Even the amount of variation between max and min is quite variable. 

As for colour, if it's not blue, turn it off. That's a simple message for kids but it's a good starting point.