What a great idea!! I think the girls will love the necklaces.
I think rings can be sized down to any size, at least in theory. The only thing that will hinder it from being sized down very small would be the settings. The settings of this particular ring look like they may be a little too wide to size down to a 4 or 5, but that's just a guess. I'm no jeweler.
In any case, congrats on your upcoming adoption day. You girls are going to look fabulous in your new bling!