Toddler and newborn

Looking for some advice. I have a 3 week old son and a 3 year old daughter in preschool. I have kept my daughter home from school since the week before my son was born to avoid her getting sick and spreading germs to the baby. She has been so sick this year! Literally every other week we have had to keep her home with illness. I feel terrible keeping her home from her friends and would love to send her back but am anxious about the newborn catching anything. Should I risk it and let her go back to preschool or wait a certain amount of time so the baby has built up an immunity?