As a server in my younger years I have dealt with this. With it being a party of six, depending on the restaurant, they may automatically add gratuity. I once served a large table that I was able to grat but there was an older gentleman paying the bill. I did mark on the bill that grat had been added, but he did add a small amount of additional grat to the check. As the rest of the party was walking out a younger gentleman from the group approached me and asked if I had been tipped enough bc of the man’s track record. You want your girls to be friends long after you are gone. Why would you put this issue between them, but it is clearly not appropriate for a 12-year-old to be in that situation. It seems like you are afraid to say no to the younger daughter. You risk alienation between the girls, and are leading the younger one into a scene she is too young for.