Does this mean that they lie on the ground and draw or that they literally use the ground as their canvas?
It could technically mean either but it’s most likely referring to the latter.
How do you know when to translate barn as child or children? I know it is probably silly obvious but I cannot get it. Thanks!
The first obvious thing to tell by is if it's preceded by an indefinite article: Ett barn ritar på marken can only mean ONE/A child draws on the floor, because of the presence of ett. Now if the sentence is Barn ritar på marken, then it's indeed not explicit whether barn in this case means child or children. But we can assume that this sentence is a generalzation, meaning children draw on the floor, as child draws on the floor is just broken English and further doesn't make much sence. In other words: If barn is preceded by ett then it obviously means a child, but when it is not, then it would mean children almost anytime, because Swedish nouns in singular either need to be in the definite form (barnet) or require the indefinite article (ett barn) in order to form a correct sentence.