Learn English – “You just keep quiet and be good”. Why isn’t the verb “to be” “are” in this sentence

I read this poem about the environment from my English textbook

The word 'be' in the sentence "You just keep quiet and be good" makes me confused. Why isn't it "are" to have subject-verb agreement?

MUMMY, OH MUMMY

"Mummy, oh Mummy, what's going to happen

If all the pollution goes on?"

"Well the world will end up like a second-hand junk-yard,

With all of its treasures quite gone.

The fields will be littered with plastics and tins,

The streams will be covered with foam.

Now throw those soda bottles over the hedge,

Save us from taking them home."

"But Mummy, oh Mummy, if I throw the bottles,

Won't that be polluting the woods?"

"Nonsense! That isn't the same thing at all,

You just keep quiet and be good.

If you're going to start getting silly ideas,

I'll take you home right away.

Because pollution is something that other folk do,

We're just enjoying our day.