
I suppose you children are not familiar with characters such as Vampire Hunter D, Alucard, Black Blood Brothers and co?
Vampire stories are VERY simple btw, really. XD
Monsters being Monster Hunters is quite common in stories.
And as mentioned above; feeding and transformation works differently. In most stories it's a type of intent that needs to be there by the one that bites or it is the idea of the one that transforms having to ingest the vampire's blood. Those are the two most common versions of transformation.
In general you should view vampire love stories with the ending of them both becoming vampires, unless the story has some weird 'un-doing' option for them both to be human. Because immortality is undeniably a curse and many vampire characters do not want their loved ones to be subjected to that.
But yeah; the happy ending is usually them both being happy together for literal eternity.
I hate different species love especially if one of them lives a long life...
It just fucking hurts thinking about the fact one will have to go earlier...
Sigh...