The temporary tattoos, that is!
I went googling and found the answer was really very simple:
We took some clear scotch tape, stuck it over the tattoo(s), and ripped it off. Don't worry, scotch tape isn't sticky enough to hurt- only sticky enough to remove temporary tattoos! Then a gentle wipe or two with a cotton ball dabbed with olive oil and... no more tattoo!
Offspring #2 is now clean and innocent looking again. Offspring #1 begged for a reprieve so the tatt's on his face are gone but he still has a few on his arms and chest. That however is for one more day only.
I love the internet most of the time but there are days, like today, when I really really love it!