All one needs is a used empty coffee can or similar, some stones, and a little water. Place can in convenient place - nail to wall for example - drop in small stones, add enough water to cover the stones by one inch. Done. Just dip left hand into can when finished and wipe, repeat until certain area clean.
When stones covered, just empty out nearest window and refill can.
No need to go back to 7th Century, just go to any Muslim nation today to see this wonderful environmentally green invention in action.
Simple fix. Dont let the kids drink pop and especially those highly caffeinated energy drinks. Then, tell them to hold it until they get home. Or, just go to McDonalds and use their facilities. Simple. KISS.