Bicycling 2.81 mph

I need help figuring calories burned!

I take my son to school then proceed to work. According to my phone's GPS it is 1.81 mi for the entire trip with .1(or less) of it being the route to my work. The entire trip takes 38 minutes (including stops) and the average speed 2.81 mph avg.

I can't go any faster than him on the way to school, and we have to stop twice to cross the street. I stop once to say a quick goodbye to him at school. From there, about 1/3 of my trip is through grass and a slight inclinec and I go much faster (although I doubt it is 10 mph!) Over the summer when I made the trip alone the it took me 15 minutes, so I could assume a speed of 8 mph.

So, to guess at calories burned I was taking the bicycling <10 mph and put in 20 minutes (half the actual length of time) I hoping that it is a fairly close representation of how many calories I burned.

Anyone think this would be a fairly accurate assumption or should I just assume that this isn't enough exercise to worry about in a day?

P.S. I make the trip twice and take the bike to pick him up at school, so my daily total miles is about 3.82; it can't be for nothing! Also, I weigh 155 pounds if that makes a difference.