Ett barn, barnet, barn, barnen.I think. Ett barn is a child and you are talking about he/she for first time. Barnet when we know that it's the child and the second time we talk about he/she. Barn is used then when we are talking about many children for first time. Barnen is used then when we know that is "the children" and if we talk about them at second time.
Sorry my english is not very good to explain things. :)
If 'Flera' is several, what would we use for many children?
I use several and many interchangeably in English, although it's just cause I'm lazy and i'm probably wrong to do so. Is there a more distinct difference in Swedish between 'flera' and whatever word I should use for many?