Translation:Twelve girls and fifteen boys are playing in the park.
tolva is the noun form, very rarely used. For instance, I might (very seldomly) refer to the twelve on a clock as tolvan, because I'm speaking about a specific instance of the number. But in general, I'd advise you always to use tolv - you're likely never going to use tolva ever.
In the numbers exercises Duolingo doesn't recognize my pronunciation of them. I get the rest of the sentence recognized (highlighted in blue as I speak), but not the numbers. I have tried to report it through the standard options, but I could only use the very generic "something else is wrong", hence why I am reporting it here hoping that a moderator will read my comment.