XtremePapers

So can I say "Excuse me, You aren't allowed to smoke in my house, for indeed it is harmful to all specially the children in the house"
By this way, you warn them and show them dislike to smoking (But is it not bad to say that guests??? )

yup.. but ther is a way to give naseeha... Nouman Ali Khan once explained that Amr bil maroof.. the wrd Amr is normally understood as command rather it includes commanding, advicing, encouraging etc..
so u choose ur wrds wisely.. such that it doesn't get on their nerves n they actually listen to u... that book 'Enjoy ur life' has good ideas on how to accomplish that.