如果只是在公园里:
The children are in the park very happily.
要是在公园里玩的话
1.The children are playing in the park very happily.
2.The children are (having fun/having a good time/having a great time/enjoying themselves) in the park.
括号里的你想要哪个都一样……
希望我的回答能帮你解决问题!
参考资料:我的大脑……