I don't know the full facts on this, but since we watercolor a lot at home, I would probably recommmend just dying the outfit the same color as the darkest stain - sorry.
My experience is that if you rinse it out immediately, you can avoid a stain sometimes, but otherwise it is fairly hopeless after the stain is set. Watercolors are permanent.
The only other trick that has ever worked for me was vinegar soak and then pouring lots of cornstarch on it and letting it sit for a couple of days. If the stain is not fully set, then there's some hope of drawing it out into the cornstarch (as it absorbs the vinegar)...and I have even drastically faded some old stains with this method.
A while back, somebody sold aprons for kids that had sleeves - like a chef's apron. They were pricey but I would consider them worth it if my kids were "dressed" for school when painting - as opposed to having them actually change clothes at home into ancient, torn t-shirts.
After a full afternoon of drying on, you are at a disadvantage to get them out, but let's hope that someone else has a miracle remedy for this and I will be watching this thread too!
who has plenty of already-painted t-shirts if you want meto send one to NY!