It should be up to the parents what organizations they sign their kids up to. This is how we get to the best outcomes, by not trying to force a single way to raise children on everybody. But let them teach their children what they value, be it god or scientific, we are in no place to judge.
You are essentially supporting the indoctrination of children. Although it would be virtually impossible to eliminate the issue entirely, it should at least be promoted to allow children to have original thought.
There is a need for a specific reason why children would need to be excluded. For example things like nudity, violence, drugs, swearwords etc. If a religion doesn't have any of that, people could rightfully protest that the ban for children was unjustified. A state need a reasonable justification.
the choice to become religious demands a certain level of introspection and consciousness, something that is hard to demand from a child. The choice to become religious is personal, and not something one is forced into by for example parents.
When parents force their children to partake in a religion the government has the chance to take their children away for example child abuse, which is already well established in most democratic countries so there is no argument made here. And if a child chooses to, there is nothing we can do.
The inability of children to distinguish fact from fiction has been strongly correlated to how much religion they were exposed to at an early age. In short, indoctrination likely retards aspects of cognitive development, much like a hammer to the head. It's therefore child abuse.