The unitary language hypothesis supports the view that parents should withhold foreign languages from their children. This is because, until the age of three, children often mix multiple languages in a single sentence or alternate English and foreign words. For example, a German/English speaking child may say, "the fire is heiss" instead of "the fire is hot." Supporters of this hypothesis believe that language mixing indicates that children can't differentiate between two or more languages.
The differentiated language hypothesis acknowledges that bilingual children mix languages, but they believe this mixing mirrors language mixing used in casual conversation between bilingual adults. When speaking to non-bilingual adults, children speak exclusively in the appropriate language.
A study of French and English bilingual children shows that language development is on par with monolingual children -- they learn their first word, their first two word combination and their first 50 words at the same age. Exposing a child to multiple languages at an early age helps them to communicate with a wider variety of people and analyze language with greater facility. Older children and adults have a much more difficult time learning a second language and seldom lose their accent or become truly bilingual.