1) I can hear birds singing happily/with pleasure in the woods.
2) They see kids playing happily/ cheerfully/with pleasure on the grass/ lawn.
这两句话就是让学习 see/ hear sb doing sth 这种用法,表示看到某人正在做某事
另外就是with pleasure 就是 with + 名词 表示状态相对于副词
例如：He told us good news with great joy.他兴奋地告诉了我们这个好消息.