Soak their fingertips in the juice of a fresh lemon. You can either squeeze the juice into a shallow bowl to soak their fingers in, or you can try soaking them directly in the lemon. Just soak for a few minutes then, sprinkle a little salt on their hands and wash with warm soapy water. You may need to try this a few times to remove the stains.
You can also try raw potato juice. Rub the cut side of a raw potato on their fingers, and then wash their hands in warm, soapy water.
ETA I'd try the potato on their faces before the lemon
Edited by Babe