You are perfectly within your rights to ask friends and family members who smoke not to do so around your children.  If your home or car are no smoking zones, this rule should apply to everyone. 

If the situation becomes tense, gently remind the people in your life that smoke is not good for children.  Even if they are not concerned for themselves, they will likely show concern for your children and respect your wishes.

If someone insists on smoking while visiting your home, ask that person to step outside until he or she is finished smoking.  If the person continues to resist or lights a cigarette inside your home, politely ask him or her to leave.

You do not have to put up with smoking around your children.  Most people will be polite enough to do as you ask, but if not, you have to be firm.  You and your children shouldn’t have to put up with another person’s bad habit if it makes you feel uncomfortable. 

If you do end up having to ask someone to leave, try to call him or her at a later date when things have simmered down to discuss the situation.  Let that person know that you love him or her but that you also love your children and feel that it is your duty to protect them from second hand smoke. 

Hopefully, the other person will realize how important the issue is to you, and will agree to honor your wishes.