Chicken pox can kill - but rarely and usually in imunocompromised people. BUT there are far more immunosupressed people about than there used to be.
In my lifetime childhood leukemia has gone from almost 100% fatal to the majority of children surviving, but they survive with compromised immune systems.
Transplant patients are another growing group, and transplants in children much more common, they were experimental.
Children with Down's Syndrome did not live past their teenage years, now life expectancy (due to antibiotics) is 40 - 50 years, but still with low immunity.
Chemo isn't just for cancer, it can be used in treating arthritis.
So if you think about taking your child into a town centre shopping will you encounter (with or without knowing) anyone who is/has:
Down's Syndrome
Arthritis
A transplant
Had chemo / currently undergoing chemo
Pregnant
In my mother's day the only one she would probably encounter would be someone who is pregnant.
Obviously it depends where you live and how you travel, in London on public transport you may encounter all of the above in one journey.