We use the Mustang PFD's too. You will see in many of my past threads that I am a big fan of them. I will tell you that they do work!!
Saw something new at West a few weeks ago. I might consider it for offshore. It is a auto inflate for kids. I think it had D rings.. cannot remember now. For some reason, this was the first time I have seen them. Made by Sospenders, as I recall.
As far as the tether and LJ's in general, I will share what we have been doing for some years: No going forward without an adult during the day. No going forward at all at night. If we are anchored, you can go without a LJ, but not forward without supervision. If we are moving, you have to wear a LJ unless you are down below. No going into the cockpit without an adult outside. Sound strict? Maybe, but my kids have not fallen off the boat yet (now the dock and boarding is a different story).
Keep them in a LJ and they get accustomed to them. Any question, just PM me.