Thanks for making this statement, Sir, but I regret to inform you may have to search for "childhood diseases" to discover it is a commonly held understanding of what they entail.
Many diseases hit children first but can also affect adults later in life. In developed countries, vaccinations for certain diseases start following birth and into school.
diphtheria
measles
poliomyelitis
tetanus
Chickenpox
Mumps
Rubella
Maybe it would be better to say, "diseases that frequently occur most often in childhood". Measles is a hot topic today and most who catch and spread it are children (I guess because of close proximity in school).
http://www.bccdc.ca/schools/Documents/EN_Guide_Childhood_Diseases.pdf
https://www.cdc.gov/parents/children/diseases-and-conditions.html
https://www.britannica.com/science/childhood-diseases-and-disorders
Have a good day.