She got a lot left to lose: her freedom. Hope. If she admits harming Kyron, that goes away, along with whatever little support system she might have left. If she's innocent, then there is nothing to tell.
The third possibility is that she was participating in another activity unrelated to Kyron that she didn't want revealed, e.g., an affair. I considered that at first, but after all that has transpired, it appears highly unlikely, unless the other activity was equally nefarious and would result in similar consequences.
Even though I believe Terri driving the baby around is plausible, I agree that it's possible she was having an affair something of the sort, and didn't want it revealed when she was initially questioned. And, while it's true that she's already lost Kaine and therefore the secret has little bearing on her marriage, she hired Houze around the same time Kaine split. I don't think it's as simple as her saying she was having an affair with X on that morning and LE smiling and saying, "Okay, that solves everything! We'll go look for the real perp now!"
Perhaps Houze knows where she was that morning, if not driving around with the baby, but would he tell her to come clean? Or would he wait for charges, bring out the person with whom Terri was having an affair, and have that person testify under oath as to Terri's whereabouts for the previously unverified hour to hour and a half, his ace in the hole in terms of defending her, IMHO.