Last time we went they had me listed as a man...I had to literally pitch a fit to get them to change it. I called and spoke to three different CM's who all told me it would be fine. I even called Memphis International and the lady said she was "fairly sure" it would be OK. But I just wasn't comfortable with "fairly sure". In the end, it was good that I did pitch a fit. Atlanta wanted me to change seats so a couple could sit together and I was relaying the story to the attendant while we were waiting for the change to take effect. She told me that I more than likely would not have been allowed on the plane (at least in Atlanta). Whether or not she was right, I don't know, but she made me feel better about getting it changed.
But you're talking about a child so it might be a completely different set of circumstances. For me, they had to cancel the current ressie and re-do it (which is probably why they didn't want to in the first place). Go with your gut...if it bothers you, have it fixed. I'm glad I did. Good luck!