There is no documented way, you'll have to use your own editor. But you can derive from the standard editor class. Here is an example of such a hack:

Define the custom editor attribute like this on the collection property:

[Editor(typeof(MyCollectionEditor), typeof(UITypeEditor))]
public List<Child> Children { get; }

With this editor code:

// CollectionEditor needs a reference to System.Design.dll
public class MyCollectionEditor : CollectionEditor
{
    public MyCollectionEditor(Type type)
        : base(type)
    {
    }

    protected override CollectionForm CreateCollectionForm()
    {
        CollectionForm form = base.CreateCollectionForm();
        var addButton = (ButtonBase)form.Controls.Find("addButton", true).First();
        addButton.Click += (sender, e) =>
            {
                MessageBox.Show("hello world");
            };
        return form;
    }
}

The add button is a simple Winforms button, so you can do anything with it.