"My children are running."
Translation:Meine Kinder laufen.
Yes. Though laufen can also mean to walk.
Can someone clarify why the sentence is "Meine Kinder laufen" instead of "Meine Kinder sind laufen"? Is there a way to know when sein is/is not required?
When you congugate a verb you drop the helping verb. In situations when you need helping verbs, they are implied. You depend a lot on context. (sie meaning they, you, and she. You need context)
There is often more than one correct answer. Meine Kinder rennen is also a possible and correct translation and is also accepted.
Unfortunately we can't see what the other choices displayed were, so no one can help you understand why each of the others was incorrect.
The choices for multiple-choice exercises are automatically generated, so nobody knows which three options you saw, or what the second of those might have been.
Because they use different verbs. Only one has the meaning they asked for.
mein is used before masculine or neuter nouns; and rennt is used with a singular subject or when the subject is ihr -- but Kinder is plural. So it has to be Meine Kinder rennen.
They should accept "Meine Kinder rennen" a correct answer. I don't see any problem of it...
They should accept "Meine Kinder rennen" a correct answer
That is one of the accepted answers, and has been for at least a year, as far as I can tell.
If it was not accepted for you, I would need a link to a screenshot to be able to say more.