Disclaimer: I am not a native Russian speaker.
The conjugation is «читают», the plural, meaning that the girl is part of the subject of the verb. I hear/see this kind of construction a lot, for instance, «мы с мужем говорили об этом» for "my husband and I were talking about it". The literal translation would be "we with husband", which doesn't make sense in English.
I'd add that saying "Мы с..." instead of "Я с..."/"Я и..." is very Russian ))
It is surprising to see that this is not the suggested answer. This is instrumental course. Without correct translation, how supposed to learn that case which is non existent at other languages.
Does the use of the instrumental case here mean, that they read together? Or how do I say, when I enter a library and only a boy and a girl are reading there, but seperately? Is instrumental still used?
I would like to know if this sentence differs from the Duolingo sentence, too. :)
Does this necessarily mean the boy and the girl are reading the same book, or could it mean they're reading separate books together?
Question (sorry, can't reply on the app): is "девочка с (мальчик correctly declined) читают" an equally acceptable phrasing? (In general; aside from not really matching the English sentence here)
If not, any reason beyond idiomatic phrasing and/or maybe pleasing sound arrangement?
Девочка с мальчиком? Yes, absolutely. It sounds a little bit less common, but not as unusual as "I now pronounce you wife and husband".