Any one here - bead loom question

Ok - my 12 yo DD has decided she wants to learn to use my bead loom. No
problems teacher her the basics on my part. But with my pocketbook feeling a
bit pinched; what would you use for thread - she's been using sewing thread
for now and I have plenty. But soon she's going to want to make a something
she's not going to pick apart right after. (btw that has been her decision,
not mine - to start over repeatedly)

So - what's cheap enough for me to buy in bulk that would still last when
she finishes a piece.

Thanks

Cheryl