In Mandarin, 小朋友 xiao3 peng2you3 (literally "little friend") is the standard address for a child you don't know (and even can be used with ones you do know, especially groups of children). 小妹妹 xiao3 mei4mei4 and 小弟弟 xiao3 di4di4 ("little little sister" and "little little brother" respectively) can be used with individual children of the respective genders. Specifically for babies, I usually hear 小baby (xiao3 baby, sometimes written as 小BB). I don't know of any terms that are not friendly; I haven't been exposed to any.
What are the terms used in your area/language? Does it depend on the age of the child (baby/toddler/young child/older child/teen)? Polite and less polite terms?