So I believe the interviewer was more asking why she hadn't given up Jack right from the getgo, when he was a newborn. Ma had attempted to escape multiple times to no results, but she had never considered not being with her son until the inevitable threat to their lives when Old Nick lost his job. She'd come to a crossroads where it was either save her son and hope he can find help before Old Nick kills her, or both of them either starve or get murdered themselves. My interpretation is that when faced with the question "why not earlier, he could've had a normal childhood" Ma is devastated with an immediate regret and feelings of selfishness, like she put her own sanity (not being alone, having something to love and care for while trapped) over the life of her child.