Thank you for your bug report regarding the issue with Serializable fields becoming unselectable. After further investigation, we have found that this problem originates from your custom property drawer.
Parts of IMGUI including the ReorderableList are stateful, which means they retain data about the SerializedObject and SerializedProperties. In your property drawer, you are generating a new SerializedObject every time and disposing of it right away. This process invalidates the state, which causes multiple drawings of the list, and also disrupts input states.
I made a small change to your property drawer to address this:

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(ChildScriptableObject), true)]
public class ChildScriptableObjectDrawer : PropertyDrawer
{
SerializedObject m_SerializedObject;

public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
{
position.height = EditorGUIUtility.singleLineHeight;
var objectFieldPosition = EditorGUI.PrefixLabel(position, label);

property.objectReferenceValue = EditorGUI.ObjectField(objectFieldPosition, property.objectReferenceValue, typeof(ChildScriptableObject), false);
position.y += position.height + EditorGUIUtility.standardVerticalSpacing;
if (property.objectReferenceValue != null)
{
m_SerializedObject ??= new SerializedObject(property.objectReferenceValue);
var prop = m_SerializedObject.GetIterator();
EditorGUI.indentLevel++;
bool children = true;
while (prop.NextVisible(children))
{
children = false;
// don't draw class file
if (prop.name == "m_Script") continue;

var propLabel = new GUIContent(prop.displayName);
float height = EditorGUI.GetPropertyHeight(prop, propLabel, true);
position.height = height;
EditorGUI.PropertyField(position, prop, propLabel, true);
position.y += height + EditorGUIUtility.standardVerticalSpacing;
}
EditorGUI.indentLevel--;

m_SerializedObject.ApplyModifiedProperties();
}
}

public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
{
float height = EditorGUIUtility.singleLineHeight;
if (property.objectReferenceValue == null)
return height;

m_SerializedObject ??= new SerializedObject(property.objectReferenceValue);
var prop = m_SerializedObject.GetIterator();
bool children = true;
while (prop.NextVisible(children))
{
children = false;
// don't draw class file
if (prop.name == "m_Script") continue;
height += EditorGUI.GetPropertyHeight(prop, new GUIContent(prop.displayName), true);
height += EditorGUIUtility.standardVerticalSpacing;
}

return height;
}
}