Alec heard Ivy screaming, but he was able to recognize with a reasonable degree of accuracy when it was pain or when it was anger, and she sounded angry, so he wasn't too worried.
Xander just shrugged and waited for further instructions. What was wrong with letting her scream it out because she was mad she couldn't do something she was expressly told she couldn't do? She was completely safe. She could see she wasn't being ignored or abandoned, and he wasn't telling her she couldn't be frustrated (the old, "keep crying and I'll give you something to cry about" routine was one he was familiar with and despised), so, let her vent her frustrations as a one-year-old. Eventually, she'd have to find out that sometimes being told no was for her own safety - like accidentally getting her hand caught in a door - but for now, she'd have to deal with the idea that some things were just... no. But it wasn't his kid, and it wasn't his job to try to parent her. So, unless Daizi gave him some directions in what she wanted, he was fine sitting and waiting Ivy out. It wasn't like Daizi's parenting was bad or anything, just not what came naturally to Xander, so he just did what he was told.