Children without pre-existing conditions are still proving to be unaffected by Covid-19, but in places that have had high concentrations of the virus, like the UK and New York, a new type of sickness is occurring, and only to children.
It is a type of body-wide inflammation, and seems to only be affecting children after they have had the virus (they are being tested, and found to have the anti-bodies that people get after they have had the virus, and as such they are showing up as negative for the active virus).
It is still only new, and there are still a lot of questions as to what it is and why it is happening after they no longer have the virus in their system, but it looks like it could be similar to an allergic reaction people develop to things like insect bites or foods, in their childhood... often after they have been exposed to it once without a reaction, but their body mistakenly creates anti-bodies to it, and then they have to avoid contact with it for the rest of their lives.
These kids are in areas that are heavily infected, so keep coming into contact with the virus, but now that they have an immunity to it, their bodies might be over-reacting with the new anti-bodies, overwhelming their weaker bodies.
At least here in Australia (so far), we don't have huge clusters of highly concentrated infections, so for those with young kids we shouldn't see this happening here, but for as long as the virus exists in Australia, there is still a very remote chance a child who has had the virus (without any symptoms), comes into contact with someone else later who is infected, and could set off their immune system... so be mindful of the symptoms - a fever, rash, upset stomach, swollen lymph nodes.
(it is being compared to Kawasaki Disease, which is said to be caused by the auto-immune system attacking the blood vessels in the body, and the heart)