I also recommend getting a proper fitting for you and your child killing machine, you may find the solution to your problem is adjusting how you sit on the bike.

But I can also second using proper bike shorts. Unlike some others, however, I would suggest you start with the cheap performance bike a$$ huggers. You'll find that the thin chamois doesn't distract or bunch; but you'll also notice that its thinness allows you to really feel how you sit on the saddle. You'll be able to tell when you have your sit bones on the proper part of the saddle, and you'll notice as soon as they move from that spot too. Instead of dulling the feel of the ride, the increased sensitivity will help you find the right spot to ride.