Excuse me ...it never happened with me cuz when my brother asked me about lesbians and said ..is it like girl and girl...he was hesitating and embarrassed thinking "whether I asked something weird or not" ...then 1stly I acted casually and said yea it's a girl and girl ...so what's the prob and talked . So you can do same ... just explain the required amount and say firmly that it's related to adults and you will know in future ...don't need to make a fairy tale out of it and it's just some knowledge..you will not ruin her innocence it's not like you are explaining the whole thing 
Have you watched Dora the explorer ?