The virus is highly contagious, but needs an opening to get in. I would be very upset that dad would not of his own accord refrain from kissing them. If you have a sniffle you don't kiss to avoid spreading germs, yet here he is willing to spread a lifelong virus, however small the risk? I get cold sores, and when my kids were infants I would wear a face mask if I had one. However, I did read somewhere that most infections occur around age 7 and are not caused by adult family members with the virus, but by other kids hugging touching their cold sore and then touching others. I guess "generally" family members care more for their own offspring and are more aware of the annoyance and social impact that having this virus causes. I would ask my doctor since I have heard that if you use Acyclovir the very first time you have an outbreak it reduces the severity. I use Acyclovir cream (Zovirax) when I first feel a tingle and it often completely prevents the blisters forming. If I were you, I would look up some facts about herpes cold sores online, print it out, and hand it to dad to read with the comment that you are sorry he has to deal with these cold sores but please try to keep from spreading to your grandchildren.