Would the vampire child actually grow and mature, or would they simply stay the same age for all eternity as they were when they were infected with vampirism?
If vampire children do develop and grow, is it at the same rate as normal children, or will it be slowed by their condition?
This is something that would be good to know.
Other details would depend on how vampirism works in your world. Is sunlight deadly to them, or does it hurt but not kill them, or is it merely uncomfortable for them, or do they sparkle in sunlight? Will they have any sort of superpowers? Are they possessed of a moral compass, or are they amoral, or are they outright evil? Do they have uncontrollable cravings for blood, or merely a fondness for it that can be curbed? Can they eat normal food or only blood? Where must they get their blood if they feed on it? Is cow's blood acceptable, or must it be human? Can they be trusted around housepets?
Do they have a severe sensitivity to garlic? I suppose that one would be easy enough to explain away and accommodate, given the modern awareness of food allergies and sensitivities. Themindstream mentioned explaining away sun sensitivity as Xeroderma pigmentiosum (these kids are sometimes actually called "vampire children.") Managing this sort of thing is a heck of a commitment for a foster parent, though.