Hi, the main difference between the 20 and the 40 is:
Size of the coil - the 20 has a 8" coil and the 40 has a 10".
40 has screen backlight and 20 does not (If you ever take her on a night hunt)
20 has 2 tones vs 40 has 3 tones
40 has slightly better discrimination.
40 has bluetooth to connect to a smart app, 20 does not.
The ONLY major difference that I think a 9 year old would want and the 20 does not have:
the 40 has a PINPOINT mode while the 20 does not - and I think that will be the major deciding factor for you/her.