Vampires can look just like normal sims in their dark form. So it should not have to be scary for kids. Here is pics showing you even the teeth can look normal. I have honestly played a full on vampire game where I turned sims and then I played one where I completely ignored vampires and the game does not generate as many, those it does generate look like normal sims, if you do spot them you can either delete them in manage worlds or go and edit them and copy their normal look over to the dark form look as well. I personally think that people who do not want vampires should just not buy the gamepack, but if they truly realy need the furniture then they should accept that vampires will be in it and then play around them, edditing their dark forms or using mods to completely deactivate them. Well anyways thats just how I feel.
Normal form, sorry about her closing her eyes, typical sim doing that as I take a pic
http://i.imgur.com/BLmch7H.png
Dark form, no difference whatsoever, except the open eyes ;)
http://i.imgur.com/46nefyK.png