I've seen those at the pet store but didn't realize they fit over a perch. I'm going to take a closer look. Looks like a really cute toy and your fids obviously appreciate the present Mommy brought home!
I have one of those but its not in the cag due to the fact that my first budgie (Squeak) would push it to the end of the perch and then push it totally off the perch, the other reason is it won't fit on the perches in Lilly's cage.
But your boy's looked like they were having fun