I smelled cigarette smoke smell on my helper after she came back on her off day. Should I confront her? Can I ask her to quit if she admits?

You can asked her if she was smoking or was she with someone who was smoking. Ask it casually and nice, so that if she is smoking, she wouldn't lie to you about it. If she was smoking, tell her that she needs to quit as she is handling your kids. Offer her help if she finds it difficult to kick the habit. If it was someone around her who was smoking, ask her to shower and have a change of clothes before handling your kids of hygiene purposes.