I told my 4 year-old daughter that only adults can use make-up. When she grows up, she can use them, but not when she is still a kid. She asked me how come some other kids use the lipsticks. Also she got the kids’lipsticks once a while from those birthday parties. I told her that it is very easy for her tummy to get the lipstick which is not healthy. She knows she should not eat unhealthy food for long time. She did put red marker on her lips once, I told her the marker is poison and it is going to damage her stomach. She does not do that ever since.

You can 禁止, but don't have to be tough. Good explaination is important.