* For a newborn or very young child: 宝宝 (bǎo bǎo) This is the most common way to say "baby" and is used for infants and toddlers. It sounds like "bow-bow".
* For a child in general: 孩子 (hái zi) This means "child" and can be used for any child, regardless of age. It sounds like "high-tzu".
So, if you're talking about a newborn, use 宝宝 (bǎo bǎo). If you're talking about a child in general, use 孩子 (hái zi).