Instrumental plural (remember that with the verb być (to be), we use the instrumental case and not the accusative).
But this is an exception, usually the instrumental plural ending (for all three genders) is -ami, except for a few words like this one.
Omg I'm never going to get this. Just when I think I know a word it transforms into this thing with extra arms and stuff ^^'
This is the question to ask when someone's coat is suspiciously long and you hear giggling out of their stomach
"Children" is a plural of "child" and you shouldn't put indefinite article in front of plurals in English – think of it like that: if you aren't talking about specific group of nouns(and in such a situation, you would use the definite 'the'), a plural denotes an indefinite mass of all the nouns you name with the plural, so you don't need to also add the indefinite article – the plural takes care of it on its own. ;)