Solved !
I had to alternate the cached and uncached modes for snippet calls as one is inside another.
In my case, this is the configuration for a Wayfinder row :
$rowTpl = '@CODE:<h3>[+wf.title+]</h3>[!Ditto? &parents=`[+wf.docid+]` &depth=`3` &hideFolders=`1` &tpl=`checkbox_produit`!]';
The Wayfinder snippet is called cached ( [[Wayfinder? ]] ), so Ditto must be called uncached ( [!Ditto? !] ).
hideFolders, that ganeshXL mentioned was the right parameter to only list documents that are leaves.
The code above displays a checkbox for each document in a product category.
The name of product categories are displayed in the <h3>...</h3> sandwich.
Chunk checkbox_produit :
<div class="chkInfo"><input type="checkbox" value="[+pagetitle+]">[+pagetitle+]</input></div>