I would go with the linen if you can find it. I got some good Barbour linen thread at a local market some years ago and it is excellent. Don't want too much stretch. I guess on kids' bows the problem with rawhide being bulky for its strength isn't a problem. As I understand it, on full power bows, the string becomes too thick for using a nomal nock leading to the use of a pinch grip on the draw.
Linen, waxed and reverse twisted makes a nice project, good for learning the process of cordage making, without being too difficult.