No offense, but you are naive.
You completely missed my point. I'm not arguing for or against this. I want to know why it matters whether the child with you is your offspring, and not another child that just happens to be with you. They are asking for proof that this is your child, and I'm asking how does that matter according to the law?
On the other issue of showing receipt, I agree with you that it's not a big deal at all and not a "rights" issue (unless you consider the "right not to be offended" a civil right).