Your very FIRST step!
20 gauge guns......
In MHO,,, not the best to start a young shooter with,, UNLESS the shooter is so small that a 12 gauge is to bulky to shoulder...
The reason I say this,is the 20 gauge guns are light, coupled with the normal higher operating pressures that is inherent with 20 gauge ammo,, the FELT recoil is just awful!
There are load recipes available for the 20 that you can use that are reduced, and have a 3/4 ounce shot capacity for TARGET shooting,,and maybe dove hunting. SOME autoloaders will not however cycle these loads..
As stated above,, right NOW its impossible to find powder... Right NOW its Powder,,, a few months ago it was primers...Wads are usually easy to find...
As far as a press...
Mec 600 single stage! base model. It will out last You and your granddaughter. Many are available on EBay used,,, for cheap,, just look for one that is clean, and taken care of.. I have seen nice ones go for as little as 50.00.. Bullet proof press.
A step up is the mec sizemaster. a bit more expensive, but has a unique sizing collet that works well for ammo to shoot in autoloaders. The collet is adjustable.
Very nice press... this is what I own, in 12, 16,, and 20 gauge.
Of course then comes a powder measuring scale.... a MUST!!! You MUST purchase a Scale!!
If it were me,,, I would buy the manual,,and read. Then slowly make the purchases you decide on as components become more plentiful..
In the mean time..
Winchester makes what are called Win-Lite ammo. they are quality loads that are reduced for low recoil... If you decide on a 20 guage gun.. (really suggest a break open double barrel) with the reduced reciol factory loads,You can make sure this is something your granddaughter is going to enjoy.
I bet eventually though,, She will step up to a 12 guage gun..
I do honestly believe that a young shooter is at a disadvantage shooting a 20 gauge with steel shot for waterfowl..,, and then again,, you are back to high pressure FAST FPS with steel loads,,and the FELT recoil goes up!!