Generally, the plural of neuter nouns don't change from their singular, especially so if they are one-syllabled. See http://www.learn-norwegian.net/tutorial/2/03conjugation.htm
Ah, good point. I was only thinking of compete sentences. I tried "children reading letters" in Google translate and it gives "barn lesebrev" -- no idea why the words are fused. "There are children reading letters (on the video)" gives "det er barn som leser brevene" -- no idea what "som" is doing there. Hopefully someone can clarify.