I was shopping for a .40 a few years ago and had an XD, M&P, Sig, Beretta, and Glock all lined up. After trying them all I instantly excluded the XD and Glock because of the "feel". I wasn't sure at the time what it was, but they felt awful in my hand. The Beretta went out because the trigger pull is several miles long. The Sig felt great, as did the M&P.
I spoke to a guy at the shop and he said what I felt on the XD and Glock was frame flex. The M&P is has rails on the inside that prevent it, so it feels more like and all steel with the weight of a polymer. I'm a happy owner of an M&P in .40 and don't regret the choice at all. I use the middle backstrap, but my brother has smaller hands and feels a significant improvement with the smaller one.
I also have an LCP, and yeah the trigger is long.
One suggestion for carry gun for small hands, you might have them try some revolvers.