For example, spanking a child in order to improve it's behavior is treating a child like a donkey at best. It thus denies the child exists as a person.
The idea of temper tantrums denies the child exists, and says instead that other things exist such as 'temper' and 'childishness'.
The idea of aspies denies the child exists and replaces him with a syndrome.
The preferences of children are very commonly denied to exist. He doesn't really want that toy, just an ad told him to pester his parents. Ads exist, and pestered parents, but not children who agree with ads or who would benefit from toys.
Sometimes children are asked to pretend they don't exist: be seen but not heard, or go to bed before the guests arrive.
When a child doesn't want a vaccination, all parents acknowledge to exist is irrationally fear and irrational demands that life consists absolutely entirely of love and unicorns.
When a child doesn't want a medication, all parents acknowledge to exist is the absolute necessity of administering the medication.
When a child doesn't like school, it certainly never occurs to parents that they are dealing with a person who has a preference and a life, and perhaps should have some control over his life. Instead, all that exists to them is a ball of clay which has the potential to be an adult with the skill to run its own life, and will get there not by practicing doing that but by molding.
And it goes on and on.
Despite all this, I think it'd be highly inaccurate to say the primary problem with parents today is they haven't realized children exist.