I'm the mother of a very tattooed daughter. She got the first at 14, I saw it when she was 15. The art gallery has kept growing over the years. She was clever enough as a teen to NOT mention it to me beforehand. I think your wife's reaction was the wisest, I think your position is much harder, because of their age and your stepdad status I would suggest saying how you feel, WITHOUT GOING OVER THE TOP, and backing up the position your wife took. I'd also talk to your wife about how to react when the next tattoo appears.
As an aside my tattooed biker chick daughter went berserk when her lawyer boyfriend tattooed her name on his arm, he had it removed very quickly afterwards.