If you are the first owner of the house, the builder should have left you a can of touch-up paint just for this purpose. If not, you'll have to match the paint. You might have to chip or cut a piece of the wall from somewhere like in the back of your closet where it won't be noticed and take it to a paint store or somewhere like Home Depot/Lowes. They'll tell you they need a sample the size of a quarter or bigger to make an exact match..good luck.
You can try to wipe it down and if that does not work (which if you have a flat finish will likely not work) then you just repaint. If your finish is flat it is very easy to match and it will look great.
If you ever have kids in the house, you should never have flat paint. Semigloss is the only way to go with kids because you can get soap and water and wipe it off.
Thanks for the info. Just had a house blessing, and my little cousins ran around upstairs playing tag
