We offer this reassurance when we see our littles falling apart at the slightest little thing like their sock is "off" to big things like a faceplant. It comes out almost unknowingly- as an instinctual reaction.
Telling a child that isn't okay that they are seems innocent enough, but as a child internalizes that message, they are being taught not to trust their own feelings. That their experiences aren't real. That their instincts are off. OR if they're strong-willed, they internalize it as ...
Either way they feel misunderstood, which is a crappy way to feel.
That validation leads to awareness, which leads to acceptance, which leads to coping mechanisms, which leads to emotional regulation & restoration. THEN- you can label & encourage when they feel "OKAY" again!