What is the ruling on swearing by Allah to perform missed prayers from before puberty, and does this necessitate an expiation if one is unable to perform them?

The pen is lifted from the child until they reach puberty. Therefore, prayer is not obligatory upon you before puberty. The address is directed to your guardian to command you to pray when you reach seven years of age and to strike you for it if you reach ten. If your oath was made after puberty, and you are unable to fulfill what you swore upon, or you are able but do not wish to do so, then an expiation for the oath is incumbent upon you. This expiation is: feeding ten poor people, or clothing them, or freeing a believing slave. If you are unable to do that, then you must fast for three days.