First of all, you deserve high praise for fighting the fight! Most toddlers fight with brushing, just like they can be very "determined" or "stubborn" in other aspects of the day. It can definitely be taxing on parents, so you deserve a lot of credit! Yes, it is important to brush your child's teeth twice per day. The bacteria in our mouths (they cause cavities by releasing acid onto our teeth after eating what we eat) never sleep, and cavities can creep up over time if bacteria are left on our teeth for long enough. Brushing morning and night breaks up the bacteria so they have to spend their time & energy working on rebuilding/regrowing rather than working on our teeth. Also, toddlers are in the stage of development where they are testing boundaries and seeing what they can get by acting different ways. If they believe they can get out of a task by putting up a fight - they will repeat that behavior. However, over time, if you are showing them consistently that regardless of their behavior that their teeth are going to get brushed, they tend to "give in" and begin to enjoy it as well. On the days she is more cooperative, let her participate and "help" with brushing, knowing that you will be the one to finish. If every once in a while you miss - those really rough mornings/nights - don't fret, one miss won't do it. It's what is happening the majority of the time (consistency) that counts. If you only brush once per day, that means bacteria has a full 24hrs of uninterrupted fun before they are broken up, and are more likely over time to cause cavities.