The story is tragic, but the fault ultimately lies with the parents in this case. They had a prescription for medication that may have helped save the child's life, but they chose not to even fill the prescription.
BTW. Freedom of speech isn't an excuse, it is a constitutional right (at least in the USA). Facebook, among other social media companies is considered a platform rather than a publisher. Thus it is immune to a degree from what people on its platform spew. You might not like the speech, but you don't have a right to not be offended. Perhaps the groups should be shut down, but what other speech is later deemed offensive and also shut down?
Agree the primary fault lies with the parents and I really don't understand what led the mother to seek out advice from an internet group dedicated against vaccine instead of administering the vaccine to her child that she was given by her doctor.
While "freedom of speech" under the First Amendment of the U.S. Constitution or "freedom of expression" under the Charter of Rights and Freedoms of Canada are legally protected, neither right is absolute in either country. While the application of those restrictions differs slightly between the two countries, especially in the application of hate speech restrictions, the Supreme Courts of both countries have upheld several categories of speech that can and have been restricted.
The legal distinction, unique in the USA, of social media being a third-party "distributor" or delivery "platform", like a telephone company or courier, rather than a mass "publisher" or "broadcaster", like a newspaper or TV station, came about when the internet was in its infancy under the protections afforded by Section 230 of the US Federal Communications Decency Act of 1996. It ultimately provided immunity from liability and prosecution to nascent social media networks. Those networks are no longer small entities, but massive operations that permeate every aspect of modern society.
I happen to personally believe that controls similar to those exerted on more traditional media outlets can also be sensibly and sensitively applied to social media outlets, without it necessarily becoming the "slippery slope" that is so often trotted out as the reason for not intervening. However, that is just my personal opinion. I don't want this to become political, so we may have to agree to disagree. In the end, it is up to US citizens and their governments to decide how to govern these "platforms" in their own country. Many other countries, including the European Union, have taken a different tack. Time will tell if those reasonable limits can be implemented and enforced without creating the slippery slope.
IMO the case cited here makes a compelling argument for some type of sensible restriction for when the "speech" is not only factually inaccurate but potentially dangerous and even deadly.