Language Acquisition is the study of how children learn languages.
Children generally develop phonemes that are cross-linguistically more common, before they develop rarer ones. For example, children tend to be slow to acquire the English "th" sounds, pronouncing them as "d" and "t" and the "a" sound in American English apple is likewise difficult. It may take as long as seven years for children to learn all of a language's phonemes.
Likewise, phonotactics may be difficult. The "str" cluster in "street" is rare cross-linguistically, and difficult for children to pronounce, and they will tend to pronounce it "sreet" at first.
Children learning languages fall into two types. They either learn social words first, like "hi" and "more" or nouns, like "mama," "dada," and "doggy."
Children are also limited in their lexicon, and will extend words they know to cover similar cases. For example, the word "doggy" may be used to describe cats, cows, and tables--anything with four legs.
Children learn, 1, 2, and 3 in that order, and then suddenly learn to count.