Bilingualism and cochlear implants in deaf children
A large number of deaf babies have received cochlear implants and are commonly forbidden, discouraged, and/or prevented from using sign language, their primary and natural modality of communication.
On the other hand, nearly every parent is encouraged to use sign language with their hearing babies for more efficient communication. And, this "Baby Sign", as they claim, boosts babies' linguistic and cognitive benefits.
"The greatest irony." Illustrated by artist Maureen Klusza, 2007(?).
Neuroscience studies show that both languages (ASL and English) activate the same regions of the left brain that is responsible for language. Speech is not central to language.
Language milestones in signlan and speech are maturationally controlled; that is, Deaf children exposed to signed languages from birth acquire these languages on an identical maturational timeline as hearing children acquire spoken languages. (Dr. Pettito)
Sign language does not hinder or cause a speech delay. It is simply another language. It is no more different than a baby acquiring both spoken French (or any other spoken language) and English.
Using ASL (in the form of signlan) does not contest with English (in the form of speech), quite the opposite. That is bilingualism and interaction that boosts literacy and language development in both languages.
Thus the risks
Hearing children are born with either choices of modalities. However, deaf children are born naturally with one choice of modality (signlan), which is often oppressed.
If a non-bilingual, non-bimodal child doesn't attain speech development to some degrees, the child may be turned to learning signed language. But, it'll be long past the critical period of language development in infants and toddlers. There are sensitive timing frames of language acquisition.
Limited hearing in deaf children with cochlear implants with no access to sign language hinders the development of language to the fullest, especially during the critical period of language. After the critical period, a deaf child may likely have some difficulty with language acquisition in either English and ASL (signlan). Vicious cycle.
Natural solution: bilingualism-bimodalism
A deaf child, who receives full access to language (e.g. ASL) in visual-spatial modality, will have a normal performance with written English as a second language.
Remember ASL doesn't hinder written English development, but rather it enhances English skills as a second language. Deaf people who are highly fluent in English are bilingual in ASL and written English.
A deaf child who does well with partial hearing via cochlear implants may still benefit from bilingualism using both languages (ASL and English). Brain doesn't favor one over another. Language is language for the brain.
Bilingualism-bimodalism (even if speech doesn't thrive) is a wiser decision than monolingualism and speech alone, should speech doesn't succeed.
Most Deaf are bilinguals when provided full access to signed language at the earliest and then fluent written language follows. They are quite intelligent, beautiful and vibrant. Embrace diversity.
Recommended further readings
Psycholinguist Dr. Francois Grosjean's The right of the deaf child to grow up bilingual.
ASL/English Bilingual Programming and Early Childhood Education: Frequently Asked Questions by Laurent Clerc National Deaf Education Center, Gallaudet University.