"Several children are swimming."
Translation:Verscheidene kinderen zwemmen.
I might be wrong, but in "verscheidene kinderen zwemmen" the verb "zwem" is conjugated for the plural children: "many children (are) swim(ing)."* In "verscheidene kinderen zijn zwemmen" it is the zijn (ben) that is conjugated, leaving the swim verb in its infinitive: zwemmen. It would literally translate to "many children are to swim."
*As far as I know, in the Dutch language it is fairly uncommon to differentiate between continuative tense and progressive tense. Although, according to Wikipedia's article on continuous and progressive tenses, you can form a sentence using zijn by adding "aan" and making the final verb a noun. In this case it might be: "verscheidene kinderen zijn aan het zwemmen" being literally "Many children are at the swimming." To me this doesn't sound correct but this is what is says.