so, "I hear that you want to simply be added to my account, but considering where the marriage has been headed lately, I don't think that works for me (also another very good phrase, btw -- "that doesn't work for me" . . . )
"I do think it's wise that we be fully transparent and accountable to each other on all joint family expenses, so I have decided to open up a joint account for this. We can add up the family's monthly expenses, and then each contribute to this account proportionally, based on our incomes. I understand that your commissions are just starting, so we can figure out a way to do this that's fair. Maybe we got '90/10' for now and then adjust it quarterly or something, based on where your quarterly income ends up. We would each have full access to this account, and full accountability to each other on making sure the kids' needs and reasonable wants are being taken care of."
If she argues with you, use the "well, that doesn't work for me," and end the conversation. Try one more time in a day or two and then just go ahead and set it up. Frankly, it's what a mediator (and a judge) is going to require anyway, most likely.
