You can use one of these! They are either called あいうえおひょう or ひらがな表.
Here’s an example:
The general format for any letter is (word)の(letter).
The Japanese word for ant is あり and so it’s used for あ in the poster.
To say あ is for あり you would say「あり」の「あ」。
Hope this helps!
Similar to the one above but some words have rendaku.
「もっとあいうえおひょう」This one fully focuses on rendaku and other fun combinations!
Edit: I realized afterwards that you already have teaching material so you don’t have to use the poster but you gotta admit it’s cute though Also, to lessen confusion, I replaced the original poster (the one with the duck as あ) with the current one above because the duck one used rendaku words. Thanks for pointing this out @seanblue! Lastly, I apologize for the large picture size. I couldn’t figure out how to make them smaller without compromising on readability