The back teeth coming through can be so painful for some kids, my little boy was in a lot of pain, i had to give him infant paracetemol for it and put Bonjela on too. If she is in a bit of pain then i wouldnt worry too much about brushing them for a bit, just avoid giving her sweet things and especially squash to drink. Maybe just let her brush her teeth a little while you do yours and then swap like the other person said. I wouldnt try to make her but if she lets you even for a second give her a sticker or some reward. Good luck