The important thing is to give them a choice but be firm on the original requirement. Teach them that some things in life must be done (like brushing one's teeth) but they still allow for some level of autonomy, which I think is an important lesson to learn. They're going to encounter this again in life and if they are able to cope by focusing on the choices between the smaller details of a larger, more difficult or unnapealing task, that's a win in my book.
