You have several sets of germs. You have your germs (from work, school or the hospital), you have the daycare and you have the sitter. Between them and things like going to the grocery store, you're going to have someone bringing home something. Has it been worse since you started school yourself? Could you be the vector, given your degree?
If your children are prone to certain infections, talk to the pediatrician about them. Do they need different antibiotics? Do they need tubes in their ears? Does everyone need to wash hands when you come home? My sister was always cold to BAM! - bronchitis. She just has a poor respiratory system or something.
For the record, my DD used to go to daycare and now she's home with me most of the time. She still gets colds from us or from her sister (whatever's at school) and returns from breaks seem to be worse - Johnny visits his cousins for Christmas and whee! new germs for the whole class! It's just the way it goes. My stepson seems allergic to February. Always has been. Always gets very sick in late Jan/Feb. No idea why, really, other than maybe he gets worn down. Make sure they get plenty of rest.
I think at the end of the day, you need to take classes and you need to work and you need childcare. You can put them all in the same germ factory, but I would expect these illnesses for a while.