Assuming that the MyPage is the root of your database, please use the following lines of code:

val rootRef = FirebaseDatabase.getInstance().reference
val messageRef = rootRef.child("message")
val valueEventListener = object : ValueEventListener {
    override fun onDataChange(dataSnapshot: DataSnapshot) {
        for (ds in dataSnapshot.children) {
            val id = ds.child("id").getValue(String::class.java)
            val text = ds.child("text").getValue(String::class.java)
            val time = ds.child("time").getValue(String::class.java)
            Log.d("TAG", text + " " + id + " " + time)
        }
    }

    override fun onCancelled(databaseError: DatabaseError) {
        Log.d("TAG", databaseError.getMessage()) //Don't ignore errors!
    }
}
messageRef.addListenerForSingleValueEvent(valueEventListener)

And the result in your logcat will be:

hi user 2020.06.07/08:22:46

Map data = remoteMessage.getData();

String id= data.get(“id”);

You can parse the body from your RemoteMessage.

My Firebase data structure is

MyPage
    message{
        -M9CrzvBuz1EBojnK9xP{
                  id: user
                  text: hi
                  time: "2020.06.07/08:22:46"

But I don’t know how to get id, text and time. How can I get it with Kotlin?
The number of children in the message will continue to increase. Can I get them all?