Hi, Teresa. It's great that they are eating on such a good routine and sleeping well at night. I'm sure the evenings are tough since they are having naps on the short side. Are they absolutely tired for the first nap at 7:30? While many babies will sleep very well soon after waking from the night, it certainly isn't always the case. I would venture to guess that they might need just a bit more A time then, even just 10-15 mins might help unless they are close to becoming overtired. And since they wake and have more A time before the next E, you have to take that into consideration when sorting out the next S time. After that first nap, they are up for 2.75 hrs, and that can be alot at 6 mos. That could be why they are only sleeping 45 mins for the next nap, and then are overtired and find it hard to sleep for the following naps. If we could just tackle getting the first nap back on track, I'd say the rest of them would be better since sleep begets sleep. You are doing the right thing by putting them to bed early to let them catch up on sleep.
As for the nanny, I would just simply state that this is their routine for the day, and such and such happens at this time and that time, etc. I wouldn't leave it open to interpretation, just state it as a fact. Explain that this method is what you have choosen to raise your children and it is in their best interests that certain things are done certain ways. Let her know that BW is a celebrated method of learning about and responding to a child's needs, and is a great tool for keeping stability and familiarity in the child's life.
I'm going to ask for help under the Multiples and Naps Forums and see if we can get some more repsonses.