Order by title Child Pages displayed in a Parent Page

Take a look at the documentation of wp_list_pages(). You can use the argument sort_column to specify the ordering. Use sort_column=post_title to list the pages alphabetically.

So your query for the child pages will become something like this:

$childpages = wp_list_pages( 'sort_column=post_title&title_li=&child_of=" . $post->post_parent . "&echo=0' );