Girls, hello. Can you tell me how to strengthen children's immunity? M…

Girls, hello. Can you tell me how to strengthen children's immunity? My kids have been sick since the beginning of autumn. They've been sick for a week, healthy for a week, and then they get sick with something else the next week. And it's been like that for four months now. I'm afraid to bathe my youngest because after every bath we treat the phlegm. I'm already chronically exhausted. I can't fall asleep. I don't even remember when I last slept normally. My head is spinning like I'm under something. Suicidal thoughts occur.