At good schools, parents are quite actively involved. I think it quite important to politely and respectfully contact the teacher and ask if there is a problem and what you can do to help.
I believe strongly in supporting teachers, but I also know not all teachers are angels, so it is a good way of quietly checking out the situation. Is the child misbehaving? (probably.) Is the teacher being unreasonable? (probably not.)
At our schools, teachers now have school e-mails, and parents are encouraged to keep in touch. This way is an excellent way, as emotions can be kept to a minimum and the e-mail can be read over before being sent.
I would e-mail "Dear Ms J., Betty Sue told me she got two red cards this week. Is there something I should discuss with her? How may I help in this situation?"
Simple, non-accusatory, helpful.