Translation:He is the child, she is the adult.
Not a native, but I am pretty sure that sometimes an implied infinitve can be omitted after use of certain modal verbs (sollen, wollen, etc.)
EG: "Ich kann nicht Deutsch sprechen" (I cannot speak German) can be abbreviated to "Ich kann nicht Deutsch" (literally "I cannot German") and have the same meaning. In this case, I think that Duolingo just accidentally a word.
I think because it says 'sie die Erwachsene' it can be translated as 'she the adult' or 'they the adults'. Seems like die Erwachsene is both the feminine form and the plural. I think Erwachsene can also be masculine if the adult is male, so der.
So it could be 'er der Erwachsene', 'sie die Erwachsene' or 'sie die Erwachsene' and mean three slightly different things.
The plural of Erwachsene is Erwachsenen. but Erwachsene can be either feminine or masculine, according to http://dict.tu-chemnitz.de/dings.cgi?lang=en&service=deen&opterrors=0&optpro=0&query=adult&iservice=&comment=&email=
When I listen to the normal speed it's like I'd hear quick "ist die Erwachsene". In the slower one there is not "ist". I anyway feel there's a mistake in this sentence, or is it two different ones? 'Cause if you say the kid is the adult it should be "Er ist das Kind, das die Erwachsene ist." Can someone explain if this sentence actually works?
///EDIT: I actually realized this now. "sie die Erwachsene" doesn't mean the the kid, but some female and just lacks the verb 'cause it's possible in German(?): "He's the kid, she the woman." You can imagine someone pointing at two people and telling who's who!