These sturdy silicone shoelaces are incredible. They are super easy to put into the shoe laces holes and would fit any size shoe, adult or child. I used one pair on my son's size 12 (kids) shoes to replace the fabric laces and the other pair on my shoes (womens 8). My son is very active and these fasteners have not budged. Having these in his shoes while he's at school helps me to know he can focus on school work and playing, rather than tying his shoes every hour.
https://www.amazon.com/Shoelaces-Adults-Sports-Silicone-Waterproof/dp/B073TR1MQ2