LED wiring questions with 9v battery as power source
It's good to hear about family projects :)

I'm not sure I understand your description of the circuit topology.  I'm assuming that for each 9V battery, he will have four strings of LEDs in parallel to each other, and that each string consists of three LEDs wired in series.  Is this correct?


First, you have to add up the Forward Voltages of each string, and make sure the total is lower than your power source (9V).  If there are any string that total more than 8.5V (or so), you'll need to arrange them in a different way.
Then, you need to figure out your desired current for each string of three LEDs.  The datasheets will show what the acceptable Forward Current levels are.  Since all three LEDs are in series, they each flow the same current.  So you want to limit this current to lowest-current LED.  Since each LED in a string is carrying the same current, it won't matter which order you put them in.
Now, for each string, add up the forward voltages, then divide that sum by your desired current.  This will give you an initial resistance value.  It's wise to use a larger resistor that you calculated, and then decide if you want to lower it later by measuring the actual current with a multimeter.
You should use one resistor per string of LEDs.  Their forward voltages aren't going to match exactly.  If you use only one resistor, and you get a large enough mismatch between strings, then you could burn one of them up.
Regarding the switch, it won't effect the performance of the LEDs.  It will cause the battery to drain a bit faster.  Basically, the switch is adding one more LED and a resistor in parallel to your circuit.
And finally, regarding battery life, that really depends on your LEDs.  Lets assume you drive them at 20mA.  If you have four strings in parallel, this will take 80mA all together.  A typical 9V battery has around 500mA-hours of capacity.  Divide 500mA-h by 80mA, and you get over six hours :)


I hope this helps!