它按预期工作：https://pl.kotl.in/D2U_9OP7f

data class ItemsItem(
    var children: MutableList<ChildrenItem?>? = null
)

data class ChildrenItem(
    var children: MutableList<ChildrenItem?>? = mutableListOf(),
    val name: String
)

fun main() {
    val itemList = listOf(
        ItemsItem(
            children = mutableListOf(
                ChildrenItem(
                    name = "childItem",
                    children = mutableListOf(
                        ChildrenItem(name = "first"),
                        ChildrenItem(name = "second"),
                        ChildrenItem(name = "third"),
                        ChildrenItem(name = "four")
                    ),
                ))
        )
    )
    println("Before $itemList")

    itemList.forEach { itemsItem ->
        itemsItem.children?.forEach { airportItem ->
            airportItem?.children?.sortBy { it?.name }
        }
    }
    println("After $itemList")
}

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测试结果：
在[ItemsItem（children=[ChildrenItem（children=[]，name=first），ChildrenItem（children =[]，name=second），ChildrenItem（children=[]，name=third），ChildrenItem（children=[]，name =four）]，name= childrenItem）]之前
在[ItemsItem（children=[ChildrenItem（children=[]，name=first），ChildrenItem（children =[]，name=four），ChildrenItem（children=[]，name=second），ChildrenItem（children=[]，name=third）]，name= childrenItem）]之后