Translation:The girls and the boys are eating a sandwich.
In English you would say ‘the boys eat their sandwiches’ whilst in French, we would say the equivalent of ‘the boys eat their sandwich’ meaning they have one each. Another example: in English ‘the men enter with their hats on their heads’ but a French person would say the equivalent of ‘the men enter, the hat on the head’. What would you say in Danish?
the boys eat their sandwiches = drengene spiser deres sandwiches
the men enter with their hats on their heads = mændene går ind med deres hatte på hovedet / mændene går ind med hatte på deres hoveder
As a native danish speaker, those are the two ways I would translate that sentence.
Danish (like German) doesn't really make the distinction between "eat" and "are eating" in normal speech.
If you wanted to translate literaly, then "Drengene og pigerne spiser en sandwich." would be "The boys and girls eat a sandwich.", while "The boys and girls are eating a sandwich." would be "Drengene og pigerne er i gang med at spiser en sandwich." (or something along the lines) (= 'are busy with eating').
But the latter normally is used only to emphasize the action, not to simply describe a situation.