Junior Mode in the Words Education app means that some letters appear on the screen as hints.
To do that in a custom album, you need to put parentheses around the letter or letters that you wish to appear as hints. E.g., '(p)arrot' will be presented on the screen as 'p' followed by five blank circles.
You can use the step-by-step instructions in this PDF guide to Words Education to create your own custom Words Education albums (which may feature words of varying difficulty in different languages or any other kinds of visual prompts).
Also please check out the custom albums that other Osmo users have shared.