Translation:The children pretend that they are Vikings.
In this specific context, children playing pretend, it's not uncommon to hear "the children play doctor" or "the children are playing cops and robbers". My first instinct was to say "the children are playing Vikings", but obviously that's not going to agree grammatically. "The children pretend that they are Vikings" is an acceptable translation, but I wouldn't actually say it in a conversation. Not the best example for an educational tool I don't think.
Actually, we accept a wide array of answers for that reason, including "The children are playing Vikings." There are 37 accepted translations in total.
The preferred answer, 'The children pretend that they are vikings' was suggested by numerous native speakers.
The preferred answer is grammatically fine, but is neither the most faithful nor the most natural translation of the Swedish.
If my children were engaged in the activity described by the Swedish, the thought that would immediately spring to mind as a native speaker of British English is:
The children are playing at being Vikings.
Pretending to be Vikings just doesn't convey the same nuance of playfulness at all, since it's not hard to imagine a situation, such as a school project, whereby children pretended to be Vikings without there being an implication of being at play.
Literal translation would be "the children play that they are Vikings", which would be understood in English but not sound 100% natural. Personally I would say either "the children pretend to be Vikings" or "the children play at being Vikings" (though that last one sounds quite stilted or old fashioned sometimes)
The children play at being Vikings doesn't sound the least bit stilted or old-fashioned to me. This is (still) the most natural way of rendering the Swedish in British English.
It is more common to say "The children play LIKE they are vikings. It is incorrect english grammar to use THAT they are vikings. FYI
I imagine that's one of the reasons the recommended answer is "pretend that they are vikings". The other alternative in English would be "play at being vikings", though that's different enough from the Swedish sentence that it might not be accepted.
Also "play like they are vikings" implies that there is a specific way that vikings play and that is what the children are doing, rather than that the children are pretending to be vikings.