I'm generally not a fan of referring to the stomach as one's tummy. I'll admit, however, that the Honey Nut Cheerios jingle wouldn't have worked as well if it had been phrased, "It's gonna tempt your stomach with the taste of nuts and honey..."
In that context, "stomach" would have sounded too stodgy--and wouldn't have come as close to rhyming with "honey." The jingle writer was wise to have used "tummy" in that case.
