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^^