First you need to identified your goals for life insurance. Is it to supplement the deceased person's income? If so for how long? What are the costs the might be incurred with the surviving spouse, i.e. child care, increase costs due to less time to cook, chores, ect? Don't forget that your spouse and child will also get SS death benefits until your child reaches the age of 18. So the goal is to come up with a figure for this in today's dollars and get a policy for that amount.
One straight forward way is, you make 50K after taxes including benefits and want to be able to keep that income until your kids graduates college say for 23 years.
Using the following calculator below*, assuming 3% year withdrawal increase, 8% return on your investment, and 10% federal tax bracket, you would need a policy for 730K, but don't forget funeral costs and a first year buffer as your surviving family settles into the reality that you just passed a way, I would round it up to a 800K policy.
If you want to complicate the situation further, say you are saving aggressively because you're a mustachian, so after every 5 years you accumulate 200k in savings. So you could use ladder of term policies for example one 5 year term for 250k, another 10 year term for 300k, another for 15 year term for 400k, and one last 20 year term for 200k. The idea being that as the years progress you would have build up investments that you do not need as much insurance.
The figures are just for demonstrative purposes and you would need to calculate the FV of 50K a year in 5, 10, 15, 20 years with your assumed rate of inflation and how much you would need from that point on.
In my case that was way too much work and got a 1M 30 term that would ensure that my wife would have enough to live on until she starts working again and some funds available for college if I pass.
*http://finance.yahoo.com/calculator/saving-spending/bud05/
