I realise the word "millions" was being used earlier to mean the majority - but just to break it down into stats for a moment.
Out of 1,000,000 children who catch chicken pox (approx numbers):
112,500 will get an infected spot (with all the risks/complications of any bacterial infection while immune system is weakened)
48,200 will develop pneumonia
33,900 will experience febrile convulsions
24,400 will experience encephalitis (swelling of the brain)
125 will die
~910,000 will be fine long term and gain immunity, but will then have a 1:4 risk of the virus reactivating as shingles in later life.
Bear in mind there are only about 150,000-200,000 cases of chicken pox each year in the UK, so a million is a lot - about 7 years' worth of infections.