Just a coil leg rope will do the trick for her .
i use the Naish ones but anything will do .
maybe a alana paddle too .

Thanks guys, does that bodyboard wrist one have enough length so that it doesnt feel like it will be tugging on her leg whilst standing?

I know it is not a really an issue for you, as you are not going to look away from a 6 year old on a SUP, but I would like to remind people reading this post that leashes should never be used on rivers. So many people drowned on SUP under the eyes of relatives because their leash got stuck in some submerged branch...

If I were you, I guess I would have make the ankle strap barely hold to the cord by some velcro or anything that would release on any abnormal traction...