They should sound like "barn-eh" and "barn-a(h)" respectively.
You could use that as well, but in most dialects 'barna' would be used.
barnet = the child (definite singular)
barna = the children (definite plural)
I thought there was supposed to be a difference between gendered nouns and neuter nouns in the definitive plural?
Most neuter nouns have the option of using either the ending -ene or the ending -a in their definite plural form.
Which sounds more natural depends on the noun and on your dialect, but -ene is generally your safest bet if you're going for a "neutral" language. "Barn" is one of very few neuter nouns where there seems to be a rather strong preference for the -a ending throughout the country.