It’s becoming clearer that one main hypothesis that scientists are looking at with B.1.1.7 is whether its advantage could be that it is better at infecting children. There are a TON of caveats, but this is one possibility that is being explored.

The idea is very simple: #SARSCoV2 appears to be a bit worse at infecting children than adults. If the mutations in B.1.1.7 allow this variant to bind more tightly to the ACE2 receptor on human cells that could essentially make it as good at infecting children as adults.

Like I said a TON of caveats. Maybe most importantly: the UK had a lockdown while schools were kept open, so you would expect the epidemic to shift more to children anyway and that of course can make it look like there is something going on with this variant in children.

Important to note the idea is NOT that the virus specifically targets children or that they become particularly sick or anything like that.
It just means that a segment of society that the virus had trouble infecting would have become accessible to it now allowing faster spread.