Make sure seams along the front of the seat have no sharp edges and won't scratch the back of your child's legs. (babycenter.com)
Although it sounds a bit more selfish than it needs to, the common formulation «I'll scratch your back if you'll scratch mine» gets the point across. (sciencemag.org)
You reach down to scratch its back in the lumbar region, just in front of its pelvis. (www2.vet.cornell.edu)