OP, 9:32 here again. One thing you could try when going on walks is to bring a stroller (single stroller if you can swing it, but double is fine also) and tell the boy that either he holds your hand or he goes in the stroller. Mine are sick of the stroller now so maybe the consequence would dawn on him and your hand would be the lesser of the evils.
Re bringing it up to MB - just ask to talk with her and then just tell her what you said in your original post, first paragraph. If my nanny said "I'd really like Joe to be able to explore and I'd like to be able to walk with him but he doesn't reliably hold my hand. Would you be ok w/ trying some sort of harness so I can make sure he's safe, while I continue to work on the hand-holding thing? " I would totally understand. Especially if you approach it as a short-term solution while he's still figuring out language, discipline, rules, etc...
I hope your MB would react well. You've been with the family for quite a while so you ought to have some credibility! Failing that, ask her to come for a walk with you and the twins!
