Bilingual children have less exposure to each of their languages than monolingual children. As a result, they never master either language fully and,

Bilingual children can acquire the same proficiency in all aspects of their two languages over time as monolingual children even though they usually have less exposure to each language. Bilingual children acquire the same proficiency in the phonological and grammatical aspects of their two languages as monolingual children do in their one language, provided they are given regular and substantial exposure to each. Bilingual children may have somewhat different patterns of development in certain aspects of language in the short term. Vocabulary is one of those areas. Sometimes, young bilingual children know fewer words in one or both of their languages in comparison with monolingual children of the same age. This is probably because all young children have limited memory capacities, and bilingual children must store words from two languages, not just one. As well, because bilingual children learn words in each language from different people, they sometimes know certain words in one language but not in the other. When the vocabulary that bilingual children know in both languages is considered together, they generally know the same number of words and have the same range of vocabulary as their monolingual peers. Most importantly, when and if differences like these occur, they are short term and are likely to disappear by the time the children begin school.

Interpersonal communication is another area where bilingual children sometimes differ. The ways of communicating in certain social situations or of expressing certain meanings can be quite different in some languages. If bilingual children are acquiring such languages and they have not had full exposure to one or both of them, then they may not have acquired the ability to express these meanings or they may not be proficient in certain social situations. Given adequate and appropriate experience with their languages, most bilingual children quickly acquire all of the social language skills and ways of expressing themselves they need.

Generally speaking, bilingual children's overall proficiency in each language reflects the amount of time they spend in each. Thus, a child who has just returned from a visit to a grandparent where only one of the languages was used, may prefer to use only that language for awhile and, thus, may appear to have lost some proficiency in the other language. This is usually a short term, temporary shift in preference that is corrected once the child is exposed to the neglected language. It is important not to overreact to these temporary fluctuations in proficiency since they are usually temporary.

Parents can best ensure that their children achieve full proficiency in both languages by providing rich experiences with each and especially with the language that might otherwise not get strong support in the extended community; for example, a minority language such as Spanish or Chinese in North America. It is important in this regard that parents who do not speak the majority language of the community continue to use their native language so that they expose their child to varied and rich ways of using language. This is difficult to do if parents use a language that they are not proficient in. It is also important for parents to maintain use of heritage languages in the home because it is part of the family culture and an important part of the child's developing identity. It helps them feel unique and connected to their families.