Can a saddle clamp be upside down on a children's bike?
Ask yourself which is stronger.  In the correct orientation a downward force would need to sheer the bolt to come out the bottom.  In the incorrect orientation a downward force just needs squeeze through the gap (bend a relatively thin plate just a fraction).  It is not designed to take force in the that direction.

And you have to consider stand over. If you get the seat low enough then your stand over will go negative.  Even if flipping worked you are going to be close or into a negative stand over.

And need room in the seat to be flipped.

Have you even tried this?

Oh, this is for light child? Light child is not likely to break a seat clamp even in a compromised orientation. Kids are not disposable but they are hard to break. Even if (and that is a big if) the rail blew out one side it is not even likely a trip to the hospital. A blow out to the front tire is more likely and will have the same scraped knee and tears.