In your child note, where you want to place your link, start by typing “[[obsidian”. You should see a dropdown menu of notes whose titles match ‘obsidian’ - and your parent note should be there. Click on it, or highlight it with your keyboard arrow keys, and press return. Obsidian will insert the full link, in WikiLinks style, which will look something like this: [[YYMMDDII_Obsidian]].
If you want the actual clickable link text to be something other than the file name, you can specify it explicitly by using a “|” character after the file name, like this:
“[[YYMMDDII_Obsidian|click here]]”.
So, in your example, if you want the word ‘Obsidian’ in your text to link to your note “YYYMMDDII_Obsidian”, your link would look like this:
[[YYMMDDII_Obsidian|Obsidian]]
I hope this helps. You can read more tips about how to do internal links in the help guide inside the app!