I was wondering about why "fler" was used her versus "flera" and found this. http://www.thelocal.se/blogs/theswedishteacher/2011/01/24/mer-mera-fler-or-flera/
So when we have "Barnen vill ha fler bullar" it means the kids want more buns (than they have). They ate all their buns and want more.
Whereas "Barnen vill ha flera bullar" means either 1) they want several buns (each) or 2)they want more, that is the same meaning as fler.
When it comes to the sentence "Barnen vill ha flera bullar" I would immediately assume that you're talking about "several" buns. It is better to keep them apart and learn that
"fler" - "more" & "flera" - "several"
"Fler" really sounds a lot better than "flera" when talking about "more"...
Hey, you might find this helpful: Fler and flera can both be used when you are comparing something, for example: det kom fler/flera elever till lektionen idag än igår Or when it is used in a context where the swedish world: ytterligare can be used, for example: Nu finns det fler/flera människor som är emot den nya presidenten. HOWEVER! If you use it in a way it can mean ''more'' it is always flera. Viktoria äter flera bullar om dagen, eller, det finns flera drömmar ( a type of cookie) i kakburken.