Parents who smoke around my kids in their home?

my parents have been having a very rough time for about 6 months now because my 29 year old brother ped away suddenly back in March. i live 4 hours away from them, but my kids and i have been going to their house quite a bit. we were there last week, for the entire week. both my parents smoke like freight trains in their home, and around anyone who comes, doesn't matter who it is. so for a week straight all 3 of my children stayed day and night in this nasty smoke filled house. now this week, they are all very stuffy and congested. my youngest is only 5 months old and his has been far worse then the other 2.. he has coughed his little head off, his nose is both stuffy and runny, and i have wiped his nose so much that it's red and sore. it bugs me like crazy that they do not get that they should not smoke around anyone, least of all little children.. however, i don't feel it's right to ask them not to smoke in their own home, and right now i can't bring myself to not go to spend time with them because they are both having such a hard time still trying to get over the loss of my brother. how do i handle this? i am due to go back for the week of Thanksgiving, but i can't stand the thought of putting my kids through this again. they have all 3 been miserable all week, especially the baby.. how do i handle this without hurting my parents feelings or without insulting them by telling them what to do in their own home? how do i handle this?