As far as lengthening the granddaughter line or reveal, I think that’d be a nice way to finish Kylo’s thesis about proving to Rey who she is. I like it.
Rey: Why would he want to kill a child?
Kylo: Because he saw what you would become. Don’t you see it? It’s always been there. Rey, you have his power.
(cuts to Rey)
Kylo: You… are his granddaughter.
(cuts back to Kylo)
Kylo: My mother was the daughter of Vader. Your father was the son of the Emperor.