Toddler sick

My 2yo has been attending school for about 8mths now & she falls sick pretty often ever since starting cc. It's like every 1-3weeks she'll fall sick once, sometimes lasting about a week :/ I would've let it be for now as I know they need some time to build up immunity but I now also have a 3mo & it's pretty hectic if they both fall sick. Any suggestions on what I can do to help her fall sick less? She takes sambuccol pretty often but not daily