as JellyKat has already said, basically your DH is right, the back & front crotch are very different shapes, to allow for better fit & comfort especially when sitting, crawling etc.
Personally I'd recommend, taking an old pair of her PJs, or jersey pants, even a small pair (something with a similar fit to what you want would be best, to save you having to make too many adjustments)
Measure the old pair, & a pair that fits now, then undo the seams of these, press flat pin to paper, news, brown or wall paper are fine, increase the size all round as needed to make them fit now, so add extra width to the center of the leg, & as above 1cm seam allowance.
When sewing, stitch back & front crotch first, then start at 1 ankle & go around the whole inleg seam to the other ankle in 1 go. :)