宝贝 is usually used in oral Chinese. It refers to little kids or someone very close to you. For example, you can call your boyfriend or your children 宝贝. It means "sweetheart" in English. 婴儿 is a more official word in writing. It is usually used in medical reports like "试管婴儿"(test-tube baby). 娃娃 is usually used in Chinese dialect. People in Chongqing and Sichuan province like to call their kids 娃娃. And 娃娃 also means toys in Chinese. 宝宝 is also used in oral Chinese. It usually refers to newborn babies and infants. But during these years Chinese people like to call themselves 宝宝 when they are acting cute. For example, a girl is scared after hearing a creepy ghost story, then she says like this, "吓死本宝宝了!" (I'm scared to death!)

宝贝 is usually used in oral Chinese. It refers to little kids or someone very close to you. For example, you can call your boyfriend or your children 宝贝. It means "sweetheart" in English. 婴儿 is a more official word in writing. It is usually used in medical reports like "试管婴儿"(test-tube baby). 娃娃 is usually used in Chinese dialect. People in Chongqing and Sichuan province like to call their kids 娃娃. And 娃娃 also means toys in Chinese. 宝宝 is also used in oral Chinese. It usually refers to newborn babies and infants. But during these years Chinese people like to call themselves 宝宝 when they are acting cute. For example, a girl is scared after hearing a creepy ghost story, then she says like this, "吓死本宝宝了!" (I'm scared to death!)

When translated from pronounce, it sounds more close to: 宝贝 (noun.) but if you want to put your emotion to the words (as most chinese do^^, this is also one point to learn chinese) then make doppelt:宝宝, it sounds more soft, sweet, also easy to speak:) 婴儿：very objective noun, without feelings, you can see this word in office news. 娃娃：typical and also traditional chinese noun for kids, it is from 娃(wa, this word made from the sounds of baby crying^ ^) but same like 宝宝， the word just doppelt, make it more soft and sweet, with love inside;) cuz chinese word is usually one syllable, so if you say: 宝(儿)，娃（儿）， no problem and also totally right, every one understand, but after hundreds of years, people now get used to use 2 syllables to build a word, only this difference^^