I am wondering the same thing. My husband and I are flying for the first time ever and taking three grandkids. We have to change planes in Atlanta with only 40 mins. to get on. I am very nervous. Glad someone thought to ask this question.
It is possible to change terminals in less than 5 minutes. I've had to do it.

Of course, I was by myself. 40 minutes could be very tight when you're with kids (depending on their ages).
Here's what you'll need to do...
* When you get off the plane, check the monitors (there are often employees helping you) to see what gate your connecting plane is at.
* If the terminal (A, B, C, D, E, T, F) is the same as what you're on, no problem, head to your next gate.
* If you need to change terminals, head to the middle of whatever terminal you're in (signage is very good), and the middle will be around gate 15 (so if you get exit at gate 20, head toward the lower number gates. Likewise if you get off the plane at gate 5, head toward higher number gates.
* When you get to the middle of the terminal, look for the signs directing you to the other terminals. IIRC, you'll turn left or right depending on where you're headed. For example, if you're on C and need to get to A or B, you'll go one way. If you need to go to D or E, you'll go the other (again, signage is very good).
* You can take escalator (faster) or elevator down to the lower level. Get on the tram or use the moving sidewalks to go to your desired concourse.
* Go back up the escalator and to your next gate.
I would not count on stopping if you have to change concourses. Make bathroom/food stops once you get to your next gate. There's a good chance the plane will already be boarding by the time you get there.