I think the problem with Regina and children are that they betrayed her. It was little Snow White, who could not shut her mouth . It was Hansel and Gretel, who didn’t want her, altough she wanted to be their mother (in a really scary and twisted way). She say it herself that she hadn’t love for a long time.
Rumple was already a father and there is nothing else more important than your own child. I think that when Snow would have to choose Charming or Emma, her choice would be Emma and also Charming would choose Emma over Snow, because the child will always be the most important person in your own live.
So Regina before she get Henry had never this emotional experience of a mother and so it is clear while she could do cold hearted send children in their death, because for her children are small little mean people, who betrayed you.
Heros don't get their Happy Ending!