I for one have never smoked but I grew up in a smoking home and as I get older I think the more sensitive I becmoe to smoke.
The other posters are right, it is an addiction, for some people. I am a registered nurse with over 25 years experience, 6 of those I worked with a lung specialist in the hospital and in his office. Some people are just able to lay them down and not even think about them again. Some can't. From my observation, it's harder for women for some reason. Although my mom just quit a year ago after smoking 2 packs a day for 40+ years.
The cumulative, long-term devestation of smoking can be horrible. And yes the second hand smoke for the kids can certainly have effects as well. My grandmother was diagnosed with emphysema a few years before she passed away. She never worked outside of the home nor did she ever smoke a day in her life! But she lived with my grandfather, a heavy smoker, for years.
As far as talking to the kids, that's tough. I agree with the other posts that you may risk alienating them. But it is not only the grandkid's health you're worried about, it's their's as well!
There are the obvious risks of lung cancer and emphysema. But there are also the yearly colds and bronchitis episodes that will take longer and longer to get over. They may not notice now because the kids are small, but they will notice at some point the difficulty in enjoying activities/life with the kids because they're tired and/or having problems breathing. And the hidden problems with their hearts and circulation systems that may not be seen for years. My step mother, as I type this, is at risk for losing one of her legs because of poor circulation. And yes she has been a life long smoker. They have even found nicotine in the cells taken during exams of women with cervical cancer!
Have them look at the money they'll save. If both are smoking, have them calculate how much money they could save in a week, month, year, if they quit.
Bottom line, tell them you love them. You are not wanting to be a meddling mother but you want them to be healthy.
Good Luck and in good health,