The hygiene hypothesis [1] is real.

Let kids play in the dirt, otherwise they'll develop severe allergies to innocuous substances. Early development is when the immune system trains on what is actually harmful. If you don't stimulate it, you wind up with pollen and peanut allergies instead of parasite and bacteria immune responses.

From the paper:

  Primary prevention
 
  Support breastfeeding, with solid foods from 4–6 months onwards

  Do not avoid exposure to environmental allergens (foods, pets), if not proven necessary
 
  Strengthen immunity by increasing contact with natural environments (e.g. by taking regular physical exercise and following a healthy diet such as a traditional Mediterranean or Baltic diet)
 
  Antibiotics should only be used in cases of true need (the majority of microbes are useful and build a healthy immune function)
 
  Probiotic bacteria in fermented food or other preparations may balance the immune function

  Do not smoke (parental smoking increases the risk of asthma in children

[1] https://en.wikipedia.org/wiki/Hygiene_hypothesis