I guess it might look a bit weird if your sims' kids are the same age or perhaps even appear older (if they were not born as vampires and did not turn into vampires thereafter) as your sims, just like in vampire movies and tv shows. I tend to make my vampire parents dress a bit smarter/more mature. I guess from a storytelling perspective, you can go super wild with vampire families. Like having one human sibling who grows up to try to cure their family of vampirism, or having two rival siblings grow their own cults and get into fights, etc. There are also vampire challenges out there. Have fun!