I don't think Annabeth had any quality time with the child, aside from trying to get her gifts, but even that detail has escaped her since so long ago. Anyways, I don't think that Lucilia would have commissioned the rune, at least not for herself. So long as Lyn's physical and mental health is okay, there is no need to constantly resort to magic just to get around things like baby talk. Lyn will grow to learn how to talk to others without needing to use a magic rune as a crutch. Can't say the same for others, but that's what Lucilia would do. Lucilia is pretty good with her Sense Motive checks, and Lyn isn't exactly the prodigy of glibness.