Translation:The two ladybugs are having their lunch in the garden.
What's wrong with saying "take their lunch" it's a particularly nice way of saying it for ladybugs.
"The two ladybugs take their lunch in the garden." seems the best translation to me.
I would go so far as to say that "are having" is just wrong. See https://www.collinsdictionary.com/dictionary/french-english/prendre
I also was marked wrong for take their lunch and don't think it should have been.