Traduzione:I ragazzi sono sulla nostra erba.
Let me disagree with Duolingo. I think that children means "bambini" (ragazzi = boys) and grass, in this case, has to be translated with "prato". "I bambini sono sul nostro prato". That's my opinion.
Your solution is correct! Often children means bambini, but, in an informal way, it can also means ragazzi. By the way, bambini is the best solution, in my opinion. Bye! :)
So, finally, we both have the same opinion...