evolution theory is taught because there's no doubt with all the evidence they can produce from it.
Yes children will decide one day, but their brains are sponges when they are young and unless corrected later, csn stick in their. There are some people who act like assholes because their parents were so they think it's ok. Even if they now know it's not ok, they still feel it to be ok. So bottom line, the teachings will effect your child, so don't assert truth to them, teach them to critically think.
Telling children about god is not bad, but telling them it's true rather then unproven is the mental abuse you've put on them. Once you drill in their head that something is true, when it lacks evidence, they just go along with it.
Honestly, in that situation if the child was dying, i think that i would. The point is that they never got to live a life of joy and happiness and telling them that there's an afterlife will give them some happiness before they pass.
Honestly, it depends on the situation. I would just leave it as "we don't know". There could be an afterlife and no god, there could be several gods and no afterlife, there could be reincarnations, rebirths or an infinite number of things. The thing we should teach them is that anything is possible, but only believe in things that has a good basis and has evidence