I don't understand. It's accusative plural of a neutral noun. Shouldn't it be "svoje děti" or "svá děti" instead of "své děti"?
'Děti' are feminine in plural.
after staraji se o is AKUZATIVE or GENITIV?
Why is the following translation wrong?
"Mothers are taking care of their children"
"Mothers are taking care of their children" is an accepted translation. If that is exactly what you wrote, you may have come up against the grading bug.
why sve and not jejich
Because the children belong to the subject of the sentence it is better style to use "svůj".