M.H.
We did the same thing when we thought I was going back to work (it was a short-lived thing). Anyways...here is how we trained ours.
We had her there everyday for a week just like you are. She and I would spend about an hour together with him in the morning. Then I would tell my son that I needed to get some work done and that he could now have Miss Elizabeth (his nanny) all to himself! (Make it special!) The two of them would play together the rest of the morning with me peaking in here and there or just getting house work done.
We would then all have lunch together. This gave me the opportunity to show her some of our "average" lunches and how much I expect him to eat, etc.
After lunch the first two days I stayed home doing more housework. The 3rd day I ventured out with the two of them to a store (this was my favorite...I was able to take my son along, but someone else watched him and helped me load groceries...my hubby isn't even that good!). The 4th and 5th days I went out for most of the afternoon. I didn't really have places to go, but I made sure I was gone for awhile for him to get used to the idea.
The first day I went back, Miss Elizabeth said he cried for about 5 minutes then he was fine. Day 2, the same thing.
I hope this helps and good luck!