If you work with a biggish group, consider huge scale material.
With the scouts I used climbing ropes, for showing how it should be tied and also allowed the kids to use it as well as smaller stuff.
I tried to offer string from 2 mm up to 10 mm, to allow the kids to find strings that they were happy with. The smaller stuff mostly for decorative knotwork, not for practical knots.
Different people like different sizes of string, different knots ask for different kinds of rope. There is no 'one solution for all' in string and rope for knot tying.
Willeke
Edited to take out a few small mistakes.