I think that there is a presumption in law that a parent will do all that can in the child's interests. It may be different in different countries. That presumption is from where all these arguments arise. Generally parents will do their best for their kids, even though others may consider their decisions dubious or wrong. Mostly in the long run those parents and their kids live with those decisions not us. Until the state picks up the pieces. So difficult decisions for people all along the line.

I consider it abuse and neglect; children lack the capacity and agency for informed consent. Access to lifesaving measures should never be at the whim of a parent.

Absolutely it is child abuse.
Parents are being prosecuted for it now.

I think the welfare and lives of children take priority over religious beliefs. Children are no longer considered property, and parents who endanger their children's lives are considerd to be unfit parents. I don't see this as beign any different.