Hi c:
Let me tell you how I did it.
1. Save your household to the gallery (in case it isn't already saved)
2. go to manage worlds
3. save your game and go to manage worlds again
4. click on an empty house and create a new family to live there
5. now that you're creating new sims click in the gallery button on the top corner on your right
6. select your family (the one you play with)
7. add the tattoos to your sim and remove all the other sims (the ones you don't want to change)
8. click Done
9. now, click in that new family (the one you just edited) and click move out
10. Now click in your family (the one you play with) and say you want to move in to that family (this will merge both families)
11. now you have, say, a dad and mom and two equal sims - one with tattoos and the other one with no tattoos
12. now click Manage household and in the pencil (edit)
13. you're now on the menu where you create your family
14. remove the one with no tattoos and you're done
15. before you go, check the relationship and see if the sim you just put tattoos on is your daughter/son
It worked for me
I hope this helped