Isn't abuse usually defined by the parent's intent rather than the actual overt act? For instance, yelling at a child, hitting a child or really any forms of punishment such as locking a child in a room can be considered a form of good-intentioned discipline if the parent is in a clear state of mind when administering it. Isn't what turns it into abuse NOT that it is ineffective from some third party point of view but that the parent commits the act out of anger or selfishness as opposed to a sober thoughtful decision on what the parent thinks is best for the child?
Religion might be child abuse if a sadistic atheist sent their child to Church as a punishment not because the atheist thought it would be a good lesson for the child but because he his mean, selfish, bad parent.
The question of whether it is bad for the children is different. Just because something is bad for children, especially only arguably so, does not make it child abuse. The parent seems to have to know, even if hidden behind an act of willful ignorance, that something is inexcusably bad for their child for it to be abuse.
Still, the question of whether religion is bad for children is interesting. However, I suspect it is not so simple. It can be broken down into at least several questions at least varying by the following:
(1) the age of the child - For instance, it must be more destructive to try to actually convince a 13 year old that Santa Claus or gods exist than to try to convince a 4 year old of the same.
(2) the degree and fanaticism of the religion - For instance, consider that many self-identified Christians don't regularly go to Church and celebrate Christmas the way non-Christians do with materialism, pagan decorations and traditional non-biblical stories of brotherhood. It must be different to send your 16-year-old gay child to a religious anti-gay group that attempts to "cure" him by beating him over the head with homophobic religion, even though that line of thinking is shown false by science, than to go to occasionally drop in to the worship center of your religion for social gatherings especially near holidays.
(3) the falsehood of the religion - For atheists like I this question is moot, but for people who adhere to one of the countless religions and sects in the world, they presumably would only suspect teaching any of the many others besides their own would be abuse since presumably teaching a true religion if there was such a thing wouldn't be bad for children.
(4) the evidence that the specified religion and/or all religions is/are false - It seems more harmful to teach a child a less delusional false belief than one that is more arguable. It seems more harmful to force a child to believe something without evidence and also something that is thus more clearly false, then to use some evidence for something presumably still false to justify teaching the child it is true. For my part, after reviewing the
evidence for and the
evidence against, it seems clear to me that there is no significant, non-rebutted evidence that any gods exist and there is evidence and convincing arguments that no gods exist.
(5) the effect of lying on children (of the specified age and to the specified degree) in general, not just about religion -This question can likely be answered scientifically. I'm not certain on the facts of it, but until someone finds those facts I might extrapolate from
what we know about shielding kids from sex and using abstinence-only sex education which while only bordering on full-blown lying I still find similar to why and how parents would lie to their children. It doesn't cause them to have less sex, but rather they have sex the same amount but get STDs and pregnant way, way more.