'Je veux' is considered rude in French and you would have to be pretty angry to tell you children or pupils ( if you are a teacher) that.
If I ever said this as a child, my mother used to retort -
"Le roi dit, nous voulons!"
meaning even the king doesn't use 'Je veux' so who do you think you are!
So that's probably why 'je voudrais' is the accepted answer.
Hope this helps!