2 There are essentially three main stages to human immune system development. The end result is a complex and highly intelligent response to pathogens. This response is most like the employment of an intelligence service to protect the body from infection. 
3 The adult immune system can produce that clever and targeted response because it has been educated by thousands of infections that it has previously encountered. It is possible to further educate that immune system with vaccines.
4 Because it has moved on to relying on that intelligent response, the aduly immune system copes poorly with something it encounters that is unlike anything it has come across before. Chickenpox is a good example, as is Covid 19.
5 Before the immune system has encountered enough infections to produce that intelligent response, it needs to have a good plan for all the infections that it has to deal with through most of childhood. As a child, essentially most infections are "novel" to the immune system.
6 To deal with the fact that every day is a school day for a child's immune ststem, when an infection comes along, instead of an intelligent response, it produces an overwhelming but less specific response. This is more like sending the army than special agents.
7 This "throw everything we've got" stage of immunity is why it is normal to have really high temperatures, heart rates and white cell counts in childhood illness. It is why it can be so difficult to distinguish uncomplicated viral infections from sepsis in this age group.
8 This stage of immune system development is best suited to novel viral infection. It's an immune system response that is more action and less thought. This is why it copes well with (uncomplicated) chickenpox and is probably why the mortality of Covid 19 is low in children.
9 Newborns are an exception to this. A baby's immune system is heavily reliant on a generous amount of maternal antibodies. As a result the baby's actual immune system can mostly afford to be on standby, relying mostly on what could be thought of as a mercenary army of immunity.
10 This is why babies are rarely ill & when they do get an infection, they have a much higher liklihood of that infection being serious. Whatever infection a newborn has made it past the maternal antibodies and the baby's own system hasn't yet started to produce the big response.
11 It may therefore be that newborns are a high risk group when it comes to Covid 19 & the lack of reported deaths in this group is explained by some other reason. Chickenpox is a seroius illness in a neonate & I suspect Covid 19 will prove to be dangerous to little ones too.
