Nonsense and silliness come naturally to kids, but
they get pounded out by norms that look down on “frivolity.”
They refers to "nonsense and silliness." You ask a good question, because it seems like
they could refer to "kids." And yes, "adults" is also a plural noun in the previous sentence. To figure this out, you must understand the context. Adults and children cannot be "pounded out," so it must be "nonsense and silliness."
Pound out is a phrasal verb that means to "make something quickly" or to "play a musical instrument loudly." (See here: https://www.merriam-webster.com/dictionary/pound%20out) However, there is a more literal meaning, which is to "smash something flat," like removing a dent from metal with a hammer. You can pound out the dent. Or pound the dent out of the metal. I believe that is the intended meaning here.
The "norms" are the societal beliefs that an adult who plays like a child is doing something bad. Society takes children's playfulness and smashes it flat over time. Eventually the child grows up and becomes a "serious" adult. Sounds pretty sad to me!
This paragraph is difficult to understand. It's a bit unusual to say that norms can pound out a child's playfulness. "Norms" don't even have arms and they don't own hammers -- unless you happen to know two or more guys named "Norm." But there are other similar metaphors in English. People sometimes say they are "ground down" like a piece of wood or metal. Or that life "chews people up and spits them out." Or societal norms can "weigh on someone's shoulders." Again, a little depressing!