My god child of 6 years old nose was blocked, so i gave her of my otrivin nasal spray for adults. now i am worried because i read that you must not give it to children younger than 12 years?

Don't loose sleep: A single application of this in a kid is not going to hurt her. These products work by strangling the sensitive capillaries in the nose to shrink the swelling. An occasional use in a kid is likely fine but I wouldn't use it on a regular basis. I wouldn't use it in an older kid or adult for more than 3 days without a 1 week rest.