The title says it all.
Mild, moderate and severe can be subjective terms. Use your judgment for mild or moderate, severe would involve serious secondary infections or issues.
Answer per child, if their experiences are different.
I checked mild / moderate. My four got it when they were between 5.5 & 1.5. The 1.5 year old had the worst case, but even he wasn't really sick, just had a huge amount of pox. Come to think of it, I think they were all pretty mild (I was thinking mild like only 4 pox, but they did get quite a few but no fevers (or maybe low, can't remember) and the pox didn't seem to bother them too much.
I had CP in 2nd grade and all the rest of my peers had it too. Everybody recovered just fine. I remember having a lot of itching and running a fever. My older sister did not get it. Vaccines were not available yet (1984). My two nieces got CP in 2006 and recovered fully so did my sister.