If she never mentioned a single issue with her marriage throughout her life and was most happily married then I don't see what's the problem with you.
She might have been happy with her married life. But that doesn't mean that it is legal to marry off kids at the age of 9. Until a girl is 18 years of age, and thereby attaining legal majority, she should not be allowed to marry. Whether she wishes to marry before that age or not is irrelevant here.
It was not considered "illegal" at that time and it was socially accepted. Even the worst critics of Prophet didn't bring this up. It was first pointed out in early 20th century.