Recently got my daughter a pair of Crocs and she found a set of charms on Amazon (generic Jibbitz). Great value with about 100 charms for $10, but on her first day wearing them one or two fell off her Crocs, resulting in an immediate crisis as we searched a friend's yard to find them. After coming home I quickly designed a washer-like ring and 3D printed it. I can snap these over the back of the charms and it prevents the charm from accidentally popping out of the holes in the Crocs.
These are very thin (1mm) to minimize any chance that they are felt in the shoe. I've also added a small hole so you can hook it with a bent paper clip or similar tool when you need to remove these. Small Crocs + big hands makes it hard to reach inside and grab these to remove -- I think the hole and hook technique might be the solution.