why not "er zijn"?
"Er zijn" was accepted for me. However, it isn't as correct as "er zitten."
Just out of curiosity, if the children were standing would ''er zijn'' be more correct?
You could say "er staat" iirc
Dzhocef : I am afraid, "Er staat" would not be correct here. It should be "'Er staan", because the subject, 'children' is plural. Besides, in Duo's translation, it is "Er zitten...", not "Er zit...".
Why not dit tram?
De-words use deze/die, het-words use dit/dat.
Why veel kinderen and not 'vele kinderen'