We have one son, who's 8 now and to be honest no one has ever given me a hard time about it (although I've occasionally had self-generated guilt). When our son was little I asked a very successful friend, also an only child, whether he thought it had caused him any problems. He said the only thing he ever learned from from being the sole kid was how f***ing awesome he was :). Do what's right for the family - your daughter will be fine.