Be real careful about "putting your foot down". . . especially when it comes to a Mother and her children! It doesn't sound like it's an issue of second-hand anything. It sounds more like setting a good example. That means going outside, or being closed away in the ba
throom, does no good what-so-ever. The children will take notice! And that's teaching you can sneak around to do things that evidently you aren't supposed to do.
1.5 is too young, they're more interested in tickles, laughter, and play, but 4.5 . . . if "asked" what you're doing, it's the prime opportunity to simply state you're working on correcting a very bad decision you once made. Then, if the interest is still there, you can explain how smoking is not a good idea, but some people do it anyway, etc., etc.
Try approaching her with what example would she rather you set . . . as her partner and friend.
Best wishes!!!!!