I have made successful osage bows with all sapwood, 50/50 sap/heartwood, a few sapwood rings on mostly heartwood and all heartwood. The more sapwood the thicker your bow will be. I will only use the sapwood on osage if I am certain it is sound and without disease. Being that your stave has been drying for two years under the bark, I would not use the sapwood because you can't be certain about it's soundness, even (or especially) for a kids bow.
The rings on your stave are not all that thin. I would chase a good back ring and if you have a problem with some violations add a rawhide, silk, linen or paper backing for insurance.