No need to notify them. They’ll know it from the manifest.
however, as previously mentioned, that cabin for this long a flight is quite boring for a young adult to play/eat/sleep alone. Don’t expect to do anything together at your seats. The cabin is simply not designed for non solo travelers. (Don’t expect even to dine together on one table!)

if if you are already locked into ANA F, then just endure it. Otherwise, better to switch to another airline or downgrade to business class.
It will be a slightly better experience for your child due to less isolation, and potentially at at a lower cost.

the YouTube trip reports show how it is.