the US doesn’t like that it could prevent children from being spanked
There’s that, but mostly some states don’t want minors to be exempt from the death penalty or life imprisonment, which would be a consequence of ratification.
(Also, the Venn diagram of those states and the ones where children can be married but can’t get a divorce due to lacking standing in court, another consequence of non-ratification, is probably a circle.)
Religion is a horrible cultural disease that causes unmeasurable harm, sure, but the USA has a well established tradition of treating children as subhuman and brutally abusing them in a vast variety of ways, many of which aren’t directly linked to religion.