句子改错。。我找！！！找不到啊。。。

The child under the tree is not his son

children改为child
children是"孩子"的复数
它的单数形式是"child"
前面主语用复数与后面的" is "和"his son"不符
所以应改为单数

The children under the tree is not his son

children是复数，而后面用了is，这是不对的。
children 变为child 或者is变为are

children-child

is变为are 或者children 变为child

children改为child