@ButchyRestingFace you sound like you work in this field or certainly have some experience.
I do work in the field.
And yes, the issue is complex. For many years, parents were discouraged from signing to their deaf children because it was thought to impede their ability to learn speech. Even within the last 5 years I know of a fluent signer (deaf parents) who was discouraged from signing to her deaf cochlear implanted toddler on the grounds that it would discourage him from speaking.
I don't agree with that view, but it's out there, and parents are still hearing it from medical professionals.
The vast majority of deaf children are born to hearing families. The child portrayed in The Silent Child is profoundly deaf, but there are varying degrees of deafness, and some children, with hearing aids/cochlear implants are capable of learning speech.
Sign language is also a complete language, like any other. It has it's own unique grammar and syntax, totally distinct to English.
And like any other language, it takes years to learn competently. Historically, sign language classes have never been free, so parents who wish to learn proper BSL (not signed English) would have to pay to go on courses, find people to practice with and keep progressing through the levels to reach a decent standard.
I would estimate I spent around £10,000 to reach level 6 in BSL. The parents in the movie appear to be affluent but this is hardly the case for many families. And if there is little in the way of an active deaf community in their area - the family in the movie appeared to live rurally - then there will be no-one for them to practice with.
And then there's the issue that most deaf schools have closed, with deaf children increasingly being mainstreamed. And the teaching assistants/teachers of the deaf they get - if they're lucky! - to support them through mainstream schools tend to have a low level of signing (salaries are poor for bilingual TAs).
In Sweden, parents of deaf children are required by law to learn sign language. But I would hope provision of free sign language courses for hearing parents/siblings is much better there than here.
Apologies for the essay! 