I don't like those Magic Erasers even on good quality paint. Best thing to do is just touch up the walls with the flat paint -- provided you have some leftover.

It's fairly normal that flat paint is not washable. Some higher end flats with a greater percentage of solids are somewhat washable, but all of them will burnish when bumped, banged into or rubbed. The great thing about flat paints are that they are usually very easy to touch up, which is why the builders leave you some in your basement. Use a roller and feather the edges of your touch-up's out by rolling from the center outward. If you use a brush, dab the paint on to try and simulate the texture that is already there for the best chance of getting the paint to blend in.

I also love the look of flat paint. I'm about to have my entire interior painted. Right now I have the original KM Frost, circa 1991, on knock-down wall finish. I don't have kids or a lot of hands on the wall, but I've cleaned up spaghetti sauce splatters many times, sometimes with 409 and it still looks good.

If you want the look of a flat paint, but that's washable, SW Duration Matte is a pretty good substitute. It's not quite as "flat" as flat, but it's pretty close.

No washable paint is truly a dead flat. If the paint film has any real washability, then it has some sheen...it may be a slight sheen, but it's there.