"他们在树上吃饭"的英文表达可以是 "They are eating on the tree." 或 "They are eating their meal on the tree."

这个表达的来源可能来自日常生活中的场景，比如在游乐园里的小朋友们喜欢在树上吃午餐，或者是在森林野餐时，人们把食物放在树上吃。

下面是一些英文例句及其中文翻译：

"The children are eating their sandwiches on the tree branch." (孩子们在树枝上吃三明治。)
"We had a picnic on the tree and enjoyed the beautiful view." (我们在树上野餐，欣赏美景。)
"I saw a squirrel eating a nut on the tree." (我看到一只松鼠在树上吃坚果。)