"The children play when the woman works."
Translation:Børnene leger mens kvinden arbejder.
At spille refers to playing a game, after rules. Like sports games, or playing cards. Also refers to (professional) acting. "Kvinden spiller hovedrollen." - The woman plays the main act.
At lege is more the free playing that children do in the streets. Also refers to playing pretend.
This is tricky because both "when" and "while" have neatly the same meaning here in English. There's a slight nuance: "when" tends to imply a habitual practice (i.e. "whenever"); while suggests perhaps only for the duration of this work. Fortunately, it appears from the comments that Duolingo accepts either "mens" or "når."