I'm glad that you've decided to go. If the kids have been treated like you said they have been, it is more than likely that the lice are dead and there is no chance that you will get it.
Almost every.single.time we picked up my stepkids from their mom's house over the 5 years they lived with her (before moving in with us this past summer), they had lice. We checked for it immediately every time we picked them up and treated them as soon as we got home. In all of those years, we never got lice.
Even last Christmas, we picked up my stepkids on Christmas Eve. Their mom did not tell us that they had lice and we didn't notice that they had it until Christmas morning. We were staying with friends for the holidays and, when my bff's SO found out that the kids had lice, he freaked out (as so many people do...as you can see by the above replies). I held my 6 year old stepdaughter as this grown man ranted and raved about how he couldn't believe they had lice and were in HIS house with it...that his dog would get it...that he had to clean the whole house from top to bottom now etc. My poor stepkids were mortified and very upset. Even though I was holding my stepdaughter quite close to me as he made a scene, so much so that our heads were very close, I never got lice (and I wash my hair daily...it's always very clean). Because of his reaction, and it being the only place we had to stay for the next few days, we had to take my stepkids back to their mother's house for the next couple of days. It was a very upsetting situation. In the end, no one got lice from the kids.
My stepkids haven't had lice in the last 5 months they've been living with us. However, they are staying with their mom for the next 4 days. I've been concerned that they will come back to us with lice so we're prepared to treat them ASAP (in fact, we may treat them regardless).
I've also been teaching for the last 7 years and have been around countless children with lice. I've never, ever gotten lice. If you take certain precautions, you should be fine. Keep your hair up. Limit close contact with the children. It's not that big of a deal.
I hope you enjoy your holidays and can keep your mind on the reason you will be together with your family and not the fact that the kids HAD lice.