Is it permissible for a boy eight years of age to enter upon a woman whose husband has died?

There is no harm in a boy entering upon a woman, for there is no need to cover oneself from a non-discerning child. If he is discerning, he may look at what typically appears from a mahram, or at what is above the navel and below the knee. Allah the Almighty has differentiated between an adult and a non-adult regarding seeking permission.