That's not how it is in most of Europe. In most of Europe, even in formal situations, children would be talked to using informal you. That's usually not out of disrespect, and most of children would be quite weirded out if you start using "formal you" with them.
I think the first time a stranger used formal you with me was when I was around 17.
In the formal address, singular and plural would both be "Warum bluten Sie?".
But as quis_lib_duo pointed out - and interestingly got downvoted for - addressing children in the formal way is not something anyone would do, neither with their own children nor when they're strangers. As long as you call those persons "children", you use the informal address; once you think they're more adults than children, you'll start to use the formal address (if they're strangers) and stop calling them "children". If they're your own children, you can keep calling them "Kinder" for the rest of their lives, but being family, they would be addressed in the informal way.
Why no mystery?
Perhaps the children have been involved in some kind of international espionage, and they are hot on the trail of a rival spy. The threat to national security is too great to give up and go play. But they also can't tell Mom, for her own safety.
They have to convince her that everything's fine. They have to lie. But the lies are stacking up, teetering precariously, threatening to undo everything they've accomplished. They are so tired of lying. So tired. They'd like to nap. They'd like to crawl into their beds and let somebody else handle it. They want to let the adults save the world this time. But they can't. They have to finish what they started.
"We fell down on the playground," they say to their mother. "We simply fell. Nothing more. We certainly didn't escape from the clutches of a foreign intelligence service, make a daring getaway through the streets of Istanbul, leap from a bridge onto a speeding train, and evade detection at the border, only to find that our own handler had turned on us. It's not like we've been up all night interrogating a turncoat in the basement. It's just an ordinary playground accident. Obviously!"
Their mother leaves, but a skeptical look lingers on her face.
The children wait until nightfall.
You're not wrong, but "y'all" should be more widely accepted than it is, and it's more widely used than one would suspect. I'm a proud supporter of "y'all". Tons of other languages have a "y'all", including German and Texan. I have a Canadian friend who added "y'all" to his vocabulary after studying Greek and finding "you" to be insufficient information while translating. Y'all should all say "y'all", y'all.
A bit long in the tooth, this series of comments. Please weed out the inane and superfluous, The cogent remarks are right on, but there's much meandering. Some of us, relying on the high deserved Duo reputation, read through all of the printed comments. Rarely does it feel inordinately extended. But ....