"The ponies are good with children."
Translation:Les poneys sont gentils avec les enfants.
"gentils" and "doux" are both good here. Neither "bons" nor "sages" works in this context, though. They're in the drop-down hints because hints are for the entire course, so not all hints are always relevant to every sentence. As a rule, the top hint will be fine (though Duo's algorithm for ordering them is sometimes imperfect).