That is one of those things I'd be really interested to hear a bit of background about.
As a general rule, I think if you are "spanking" hard enough to leave marks, that's too hard.
The point of a spanking is to get the child's attention and let them know that you mean business. Not to injure them. In my happy little world, at least.
Reading about this from a few other news sources some commentors did bring up some good points. The child could just have super sensitive skin that turns red and stays red for a long time. It's not that uncommon. Perhaps the child did something incredibly dangerous, like running out into traffic, and panicked mom hit a little harder than intended. Or perhaps she really did just hit the child harder than was warranted.
From
Texas Attorney General:
But yeah, it seems totally outrageous to me.