Scrubbing once won't make a belly button clean. A daily dose of TLC will surely do the trick. When my son was in preschool, I started cleaning his belly button with what I use to clean mine. I actually want to do it earlier, as soon as the umbilical cord fell off, but thought it can wait when he's old enough. Old enough for his skin to handle the solutions I use to make my belly button clean.
The things that I use are actually not that harsh. I use alcohol, baby oil and whatever acne solutions I have. I don't use it all at the same time. I use whatever tickles my fancy at that particular time. After taking a bath, I put a small amount on a cotton bud. Then gently scrub the creases, where dirt usually accumulates, of the belly button. A small amount with a gentle scrubbing usually does the trick -squeky clean belly button!