My girl has moved onto a 24in bike.
I've parted out her Specialized HotRock, put it back to stock and gave it to another kid. What I have here is the suspension fork, wheels, brake, tires to turn your kid's bike into a legit MTB.

SPAM: Upgrade your kid's singlespeed 20in bike!-img_1679.jpg

Full details here:

Please feel free to ask any questions and I'll be more than happy to provide info.