Yeah, we have a toddler right now too, and if I’m estimating, during the months of October - February over the last three years I would guess I’m at least a little sick half the time.
The one that shocked me was rotavirus. At the time my kids were little it wasn't part of the regular vaccination regimen here in Ontario yet, was optional and our doctor didn't tell us about it. The kids both got it at the same; wasn't a huge deal for my son who was still breastfeeding, just had some diarrhea. But my daughter is three years older and couldn't keep food down for many days and lost a few pounds was very unwell. Awful.
Both our kids had stints in an isolation ward being fed/hydrated through a tube with rotavirus at ages around 3-4. They couldn't even hold down small amounts of water for days. I could see how it kills so many kids in developing countries without access to that.