Many people have opinions about "second-hand" smoke exposure. My personal thoughts are that any amount of ETS can be dangerous for children, so I would encourage you to keep your children away from it as best you can. Although your children may be around these smokers for only a relatively short amount of time, it is likely that they are also being exposed elsewhere, such as restaurants, malls, and other public areas. Every time you and your children are around people who smoke, the opportunity exists that you may be breathing in the potentially irritating chemicals found in smoke.
Children who are exposed to ETS are at higher risk for some diseases or the worsening of current medical conditions, such as ear infections, hearing problems, and breathing problems like colds, asthma, and lung infections. Immediate effects of smoke exposure may include nausea, dizziness, and headaches; long term risks include lung cancer, heart disease, and cataracts.
Given these risks, children should avoid ETS exposure as best as possible. Perhaps the couple will agree to smoke outside before and after you and your children arrive. It is important that you ask the couple not to smoke when you and your children are in the house or around the times you are expected to be there. I hope they will understand and respect that you are trying to protect the health and well-being of you and your children.