My daughter has started peeing her pants at school- 3rd grade?

First thing first, make an appointment with your doctor to make sure there is nothing medically wrong. Kids her age can get UTI's. If nothing is medically wrong, either something is happening at school or she is wanting your attention because you now have a new baby. If she is wetting at night, I would not hesitate one minute and put her in a diaper at night. What may have to happen also is a threat of if she cannot stay dry during the day when there is nothing medically wrong with her, her new wardrobe will change from fruit of the loom to pampers size seven diapers. I know it sounds alot since you already have a 21 month and a 4 month already in diapers but hey diapers work really good these days. It would sure be better than constant changing sheets, bedding, pajamas, or wet clothing during the day, I was a bed wetter and I can sure say its better to wake up in a wet diaper than wet bedding and pajamas.