Query specific posts from parent by slug rather ID

there are two WordPress functions you could try:

http://codex.wordpress.org/Function_Reference/get_page_by_title

(as suggested by @t31os in the comments to your quoted link)

http://codex.wordpress.org/Function_Reference/get_page_by_path

if you have already tried them, please describe how these failed to do what you want.

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