How do you adjust the size? Its too small to fit over my hand and the string is not expandable?

This is how to adjust the bracelet1. Hold the mala with the knots at the top. Use thumb and forefinger from each hand to hold beads on either side of the knots. (Left hand hold bead on left, right hand hold bead on right) 2. Pull hands apart and away from the knots. You can pull pretty hard! 3. To tighten, pull small beads at the end of the string. Please let me know if you have any questions.

Would this fit a small child? (4.5 years old)

My guess is that it would be too big. I'm quite small and it's big on my wrist.

Can this fit an 8" wrist?

It can expand to be quite large or small, depending on one's wrist. I wear my mala with several others, above my Fitbit, so it's pretty far up my wrist, and it's just fine.