Grandmum and grandma share the same meaning. Why is duo lingo saying grandmum is incorrect?
Why is "granny" not accepted?
"vovo" would be more like "granny" in Portuguese. You answer should still be acceptable though.
Nan is not accepted either :/
why is it "beija a avó" and not "beija à avó"?
Beijar uses direct object. That means: no preposition is required by the verb.
So: Beija o pai, beija a mãe.
Not: Beija ao pai, beija à mãe. (Remember à = a + a = preposition + article)
We would normally say the boy kisses his grandmother, presuming it is his grandmother.