For a few dollars, the Elastic Guides Pack by Hemline just makes sense.
$3.80 (October 2019) gets you 3 elastic guides in 6MM, 12MM and 20MM allowing you to thread a variety of elastic widths. This sewing tool is great for threading elastic through narrow casings allowing you to keep the elastic taut as your thread so that it doesn't twist. The elongated flat handle is easy grasp making the whole process a breeze.
As much as I love my trusty safety pin that I've used for years, I wonder why I've waited so long to try this?
Yellow Guide: 6MM <
Green Guide: 12MM <
Orange Guide: 20MM < (I have used this to thread up to 25MM without an issue)
Here is a pair of shorts I made for my son using 20MM Double Knitted Elastic for the waistband. If you are using non roll I suggest running a stretch stitch down the centre after threading to stop it from shifting. I love double knitted elastic for kidswear because its soft and comfortable to wear.
If you end up grabbing a set, let me know what you think!