Translation:There are many children in this tram.
Actually zitten feels a bit unusual here. It makes it sound like this tram contains a lot of children. Like er zit veel fruit aan deze boom. Or er zitten veel kleren in mijn kast.
It could mean there are many children that are sitting, in the tram. But that would be an unusual thing to want to say and because of the ambiguity of the sentence you wouldnt likely say it like that. (More like veel kinderen in de tram zitten. Many children in the tram are sitting )
So this sentence sounds a bit unusual to me. But who knows maybe in parts of the Netherlands this does sound normal.