When will my kids stop getting sick all of the time 😩

I have a 5, 3, and 20 month old. My oldest started preschool in the fall. The other two are home with me. We go to playgrounds, we go outside, they play in the dirt, we have a dog, they play dates with other kids, we take vitamins, they eat a good amount of fruits and veggies for toddlers, we wash hands before we eat and after we come inside, we see our family, and we still mask indoors when in public places. I feel like these are healthy practices to build their immune systems but they get sick constantly. Pretty much every 2 weeks. Is this normal?! When will their immune systems bulk up?! I’m so sick and tired from them being sick (but not tired). Any encouraging words?