I still have contact with my ex, however, I've never felt compelled to ask him the "what went wrongs" during our relationship. To me, I don't want to leave any impression that what ever went wrong could now be fixed. He has mentioned a few things that he felt he went wrong during our relationship, and for him, he wanted some type of closure/foregiveness, and I did forgive him and that gave him closure so that he could move forward.
So, to answer your question, and I hope I am not taking this out of the relm, but I do not think it is childish or stupid to do. I believe that if you have a question to ask to find out the "what went wrongs" so that you can become a better woman and try to improve on yourself, so as not to carry any excess baggage into your next relationship, then why not. Two mature adults should be able to have these types of discussions, without any hangups or disregards. Again, I personally do not see anything wrong with it. In fact, I want to say that it should be welcomed in that I think these types of conversations should happen, so that we as women do not carry the "what went wrongs" into the next relationship.
Not only can we all learn from our mistakes, but "knowing" gives us some type of closure and understanding. And I also think that with some individuals, it can leave the lines of communication open and possibly keep the "ex" as a friend, if nothing else. My ex and I are very good friends because of this.