Here is what happens if children dont own their bodies.
The only way for you to justify ownership of children is if you only do things for their benefit.
Then many problems rise from your position.
Its not just forced abortions and forced medical actions, but also things such as brain implants become logically mandatory.
If there was a brain implant invented which prevents children from being criminals in the future and cures them of depression or any mental illness, you would have to agree to that implant, because not agreeing is statistically harmful.
Same goes for basically any implant.
By taking away body ownership from a child, you have merely allowed the government to own your child.
Thus, all freedom is lost.
Would you want for a government to put brain implants in you?