I understand that religion is detrimental for modern humans,
False, most assuredly. Detrimental? I'm unconvinced.
Sure, if the alternative was for everyone to read CSICOP and become skeptical scientifically minded individuals then sure religion would be harmful to society. However, that's not the alternative. You undermine religiosity and people remain just as `spiritual' and seek out more harmful, less stable forms of mystical thought, e.g., homeopathy, belief in spirits, medium etc...
The failure of any major world culture to exist without substantial religious, spiritual or otherwise mystical beliefs (even a semi-mystical view of the benefits of great literature or the wisdom to be found in meditation) casts doubt on your claim that religion itself is detrimental. Perhaps the harm done by religion is balanced by the comfort it gives to many people who can't or won't find the truth comforting. Perhaps religion is merely the least bad outcome of underlying psychological tendencies we all share pushing us to interpret the world as `speaking' to us and to inject emotion into our evaluation of claims. In the long run with appropriate genetic engineering it might be desirable to phase our religion but for the moment it may be better than things like homeopathy, talk about Chakras and other spiritual interests that aren't religion.
that teaching religion to children is a form of abuse, and therefore indoctrinating anyone under 18 should be illegal.
This is a very very different claim. Sure, in the long run weaning humans off of religion may be desirable but now the question is what will that child be best served by?
As much as I feel that religious indoctrination before adulthood is brainwashing people into holding certain extremely implausible beliefs in most cases it is probably beneficial for the child. Most of America is highly religious and if you deny them that cultural belonging religion provides in these area you hurt them far worse than the harm done from carrying around comforting but incorrect beliefs for their lives. Just make sure there are ample opportunities for them to consider the question later and reach a more informed decision when they are capable.
What is most important is to make sure atheist views, socialization etc... become widely distributed so it's viewed as just another kind of belief. Furthermore, tackling the very hard problem of providing community unity and socialization without any masses is important..
Atheists visibly taking roles in community service programs would be much more useful than viewing religious indoctrination (as are most lessons from parents) would be much more useful.