An abridged version of conversation today, after dinner:
Patrick: It's bathtime.
Mariel: Please, can I play dress up instead?
(she just got new dress up clothes from Lexi, and she is very excited)
Patrick: Well... 5 minutes. You can play for 5 minutes and then it is time for bath.
Mariel (thinking hard): No, Dad. I want FOUR minutes.
Patrick (straightfaced): All right, 4 minutes.

She has the idea of negotiation, just not quite the math skills. This won't work for long. Of course, at the age of 3 the amount of time agreed upon does not matter at all, as she still has no real sense of whether 4 or 40 minutes have passed.