Shoe lace clips/ anchors/ locks for keeping the shoe laces in place. I printed these to keep my 4yr old son shoe laces in check, so he no longer has to worry about tripping over them at school if they became untied.

I printed mine out of PLA and it works fine. I will do a re-print out of flexible PLA just to make sure that it is comfortable on his little feet or attach the existing ones on the outside of his shoes instead of the inside. 
Files are attached if you want to do some of your own modifications.
To use: thread your lace from the center hole first, then weave through the hole close to the jagged end; fold over jagged end and then pass through the last hole with the slot to lock in place.