First, the pics:
As you can see, Dragonair had some smudges on his tummy.
Blaziken was a bit in rougher shape, with typical paint marks in blue.
The method? Just a regular eraser! It's worth mentioning that some erasers are definitely higher quality than others, so make sure to get a decent one or that it's not too old. I usually buy my school supplies from Asian stores, and erasers are usually soft and white, and they work really well.
So get a good eraser and have at it! It's important to note that erasers will leave some residue so if you rub an area with glossy paint it will look like it leaves a matte finish, but all you have to do is rub off the residue (I just rub it off with my fingers), or try washing it with some soap and water if you like.
Whether you're touching up a collection piece or sprucing up some sales fodder, I hope this trick helps to restore your Kids!