$40 an hour seems cheap for a full sized back hoe with a skilled operator(I see you have a 4500 but not sure what size that is) They are 80 per hour where I live for sizes like 580 Case with operator. 450 dozers with 6 way blades are same rate.
If you are working by the hour for $40 and getting 40-50 feet per hour and doing the tie-in and back filling with another tractor, then I would think you should be adding additional time for your 8n if you use it so your hoe keeps working. I think you have offered your service at a bargain. Next job should be a bit higher. It may seem like a lot of money but think about your time, equipment rental rates (check local rental companies rates and compare to your personal rate) then add in cost for your labor even though it is spare time work, you should be paid at a comparable wage for skilled operators in your area. Then you need to consider fuel, wear and tear, insurance requirements, possible break down costs and I think you will see that you are underpaying yourself a lot.
DONT forget to pay the grandson for his work. I see new labor laws restrict use of children for farm labor below 16 years of age for any activity, cant even use them to gather the eggs.