ColtCup45:
You could do what I did.... I'm a 1911 guy, and kinda missed the thumb safety on my M&P40C. An opportunity to pickup a 9C turned up, and it came with a thumb safety.
I just put the 9C slide (& barrel, etc.) on the top of the 40C frame, and the 40C slide & such on top of the 9C. Poof, thumb safety on the 40C, which I carry around the house, and the 9C is in the safe or range bag.
(I intended to give the 9C to my daughter, but she insists on moving to New York City, so I'm stuck with it.... She really doesn't need the thumb safety in the way anyhow.)
To echo others, the M&P series DO NOT need a thumb safety. I find the one on mine is just a hair too easy to flip off it was on, or vice versa, but being used to a thumb safety anyway, it's a non-issue. The kid would be better off not having to bother - it really adds nothing to the safety of the gun.
(You ought to see me with holsters for the 40C. I normally carry a top-break style for my 1911's, and have the same for the 40C, but once in a while I use a Kydex joke that has no top strap. It's quite good otherwise - I really hate Kydex - but I seem to lose it when not finding the break. Then I put the leather top break back on, and can't get the fool gun out of it for a couple days....

)
I think you could add a thumb safety, BTW. I'm not sure if the sear block is the same or not, but at this time I believe that Brownells is selling only one sear block, as a complete assembly. That should include the Hilary Lock, magazine safety, and thumb safety, maybe.... I didn't take enough time to figure out how the thing interacts with the sear block in mine. About all I do know is that the ejector doesn't fall off when re-assembling the block, and the sear and sear block pins ("axles") are headed pins that only go in one way. It also appears that the Hilary Lock and the thumb safety may be mutually exclusive, but that may not be the case.
You would also have to cut a couple of small square holes in the frame, but that's trivial.
Now I gotta go take the gun apart and....
Regards,