Some sort of kid friendly peripheral? Maybe specific like bongos or general like a small big button gamepad?
A more kiddie version of Mii’s? Or Miitopia?
A kid exercise app, but more casual? Or music creation?
I guess the megaton version of this is a kid version of the switch itself, no dock, smaller, non detachable controls on sides, cheaper. But no way they kept something like that under wraps.