You should have told them "If you find my sitting here enjoying a meal with my 3-year-old child offensive, at a kindergarten event for children, then YOU move. If you don't stop harrassing me like this, I will take it up with the school's headmaster. I have just as much right to be here as you do."
Don't let them disrespect you in front of your child, and especially do not allow them to bully you in front of your child and get away with it. Don't let your child see that it is ok to disrespect and bully you - that's a lesson you do not want her to learn.