The right kids, in the right circumstances, could do it, even as young as the OP's kids. Even as young as Maxine.

She sounds like a neat little kid. And those were cloth diapers back then, too!
One thing I come back to, however, is this:
Suppose, God forbid, there was an emergency at home, and one of the younger kids was seriously injured or -- God forbid! -- killed.
The same exact thing could happen if the babysitting kid was 12, or 16, or 21. The same exact thing could happen if all the kids were with a neighbor. The same exact thing could happen if the kids were at a daycare. The same exact thing could happen if all the kids were at school.
But, if it happened in all those other situations, Mom would have to deal with the pain of the situation, only.
While if it happened at home, if the eldest was too young, legally -- then Mom would have to deal with the pain of the situation, AND would have to face legal troubles for having left them, AND maybe even have Social Services step in and take the other kids -- whether it would be fair or really needed or not.