I'd really like to get some fabric paint to make it more permanent. I just used a Sharpie. I plan on altering the girls' shirts too.
And, get this, I was reading the inside of the Coke shirts - they have some pretty wordy messages :o) - and they're made partly from plastic Coke bottles! Who knew?! It states something to the effect of "this shirt has approx. 3 20oz plastic Coke bottles in it". You'd never know by feeling them! Pretty cool, even if it's all Earth Day-ey!