Rachael,
what a super way to engage the children and make a fun lesson out of underpants! I can imagine the little pants dripping dry! I’m trying if the best underpants are absorbent or not — perhaps this could be “nappy” design, where the most absorbent won’t drip at all and the worst would drip all the water out. If the material was not absorbent at all, then the drips would all come at the beginning, and a semi-absorbent material would drip away for ages.
Did you use this template for the underpants?