Translation:The children have been playing for several hours.
The hint system isn't perfect, but it tries to serve you what it thinks is the best / most fitting translation depending on context.
Sometimes we, as the contributors are the ones to blame for not constructing hints properly. Some of these preposition hints should be connected with other words, forming a "multi-part hint". This way very uncommon translations for the prepositions won't be shown, and you'll have to use the hint for the whole expression to get an idea of what it means. We try to do this, but I know we've forgot a number of times (reviewing hints takes a bit of extra effort).
This isn't the case for this sentence, however. Duolingo often displays too many hints, and you shouldn't trust them all to make sense for the sentence in question. Unfortunately, we cannot prevent misleading hints to appear in the list.
I have come accross times when I knew the word that would best fit the sentence but after checking the hints I saw that it was the second or third translation. I think that the more I've practiced, the better a feel I've gotten for when and where to use words like, vet, instead of kjenner, and although im not perfect, I think that, in time, the same will be true of the prepositions.