let's play cops and robbers

My boss called me Tuesday evening to ask me which day of the week the 28th was, because he was without his calender. When I told him it was a Saturday I heard him swear for the first time. I thought it was strange to be upset about this but was wise enough not to ask.
Yesterday, I was minding my own business in my little corner room office, he called me up ten minutes before I left to let me know that he was talking about July and not June and had forgot to mention that and that he would need me that day for a bit longer than my usual office hours and if it would be possible for me to make no plans for that day. I was impressed by him thinking that far into the future (he's usually not planning further than 10 minutes, that's what he has me for) that I said yes without knowing what he really needs. I had an idea what it might be, because a case that we've been working on fitted nicely into that pattern. I was so wrong.
I can't really go into detail because it's highly classified information (wow, doesn't that sound cool?) but he needs me to play the resident Mary Poppins for some people.
I was fighting hard to keep my face straight. I'm not the nurturing type, that's a fact. But I don't know how I could make anyone believe that I was. Plus I may need to get myself a British accent. I obviously wasn't doing a good job at conceiling my amusement because he asked me if I thought his plan sucked. I honestly told him that it was a brilliant plan but that it wasn't so brilliant to make me a nanny. There's quite a bit of money in it for me, though. And I really don't know anyone better for the job itself, if it weren't for the nanny part.

Bottom line: if any one of you has any hints and tips on how to appear like a nanny for two kids age 15 and 17, feel free to fire away and let me know. The kids will of course know that I'm not the nanny, but some other people really need to believe that I am.