在英文中，当某些动物或儿童开心地玩耍时，可以使用动词 "play" 来表示。例如：

The children were playing happily in the park. (孩子们在公园里开心地玩耍。)
The puppies were playing with each other, chasing their tails and rolpng around on the grass. (小狗们在一起玩耍，追着它们的尾巴，在草地上打滚。)

你也可以使用形容词 "happy" 来表示动物或儿童在玩耍时的心情。例如：

The happy kittens were playing with a ball of yarn. (快乐的小猫在玩线球。)
The happy children were playing tag in the backyard. (快乐的孩子们在后院玩追踪游戏。)

还可以使用短语 "have fun" 来表示动物或儿童在玩耍并且开心。例如：

The dolphins were having fun swimming in the ocean. (海豚在海里游泳时很开心。)
The kids were having fun playing hide and seek in the forest. (孩子们在森林里玩捉迷藏时很开心。)

这些表达方式都是来自英语的常用用法，并且在日常使用中很常见。