I'll paste in your questions here and answer them individually:
1/ -I don't know what kind of insert SUPs use, can you link to some pics or explanation?
I'm not sure of the "inside the board" configuration, but what shows is a simple 8mm threaded insert, ready to take the stud on the bottom of your mast base twist plate.
2/ -In your test, you had an Aero with this mastbox extension, and you STILL couldn't make the smallest sail work, is that correct? So what do you think, could Prokids Go be made to work with 1m rigs? I ask because I can't try any boards before ordering.
I'm not completely sure, but I know with a 75 Kg. guy (my size) I could not get the 1.5 or 1.7 Retro Rippers to sail very much above a beam reach. You rake the rig back and it stalls the sail.
Probably, with an insert as far back as possible (or perhaps an insert "IN" the center fin box) and some sort of "set forward" fin you could make it work.
Looking at the photo of the 2010 Prokids Go TufSkin, you can see how far it is from the
back of the mast box back to the "handle" which is probably where the center fin box option is installed.
If you had a SUP type 8mm insert installed within a couple of inches of the LE of the center fin (the plastic 31 cm FRN fins are OK, but a little larger slalom blade set forward
on a base about 50% would be alot better) I think your little ones could sail upwind pretty well with a 1.5 m2 rig.
The weight of a 1.5 is only ounces more than a 1.0, but get the lightest mast and boom you can find as those are the major weight items. Make sure the mast is only long enough to fill the luff sleeve to the top. Any mast above the top of the head of the sail is just weight (in the wrong place for sure).
3/ -In your (vast) teaching experience, what is the age (or weight) when you can effectively teach kids on all points of sail?
A motivated 5 year old can achieve this, but probably 6-8 years old is more realistic.
Until then, put a little boom on an old sail and let them stand in front of you as you sail.
Then get a big Start board and you kneel on the front while they sail with a 2.5 m2 rig.
You can help the little ones get the sail out of the water, then if they are taught to balance the rig, they can sail quite nicely, only needing help if they drop the rig.
Hope this helps,
P.S. When you get ready, send me an email and I'll explain how to sail with them, in complete control, with a "tow board" and tether system.