Learn Spanish (Latin America): Niños, no se comporten mal en el museo. - Children, do not behave badly in the museum.

This is the ustedes command form. To give an order using an -ar verb like comportarse, you swap the usual -a or -an ending for an -e or -en. That is why the normal statement form comportan changes to comporten for a command.

Why does the se go before the verb in no se comporten?

With negative commands, pronouns must always go between the word no and the verb. If this were a positive command (telling them to behave), the se would be attached to the very end of the word: compórtense.

If this is a command for children, why use the formal ustedes form?

In Latin America, ustedes is the standard way to say "you all" for every situation, whether formal or informal. So even when you are telling a group of kids what to do, you use the ustedes command.

Why use mal instead of malo?