I politely suggest that almost anyone over the age of 8 would recognize a talking animal - whether a donkey or a serpent - as not intended to be taken literally. This is part of the problem with arguing with creationists: you guys are literally begging to be ridiculed, with your rejection of the findings of mainstream science and the necessary associated embrace of absurd conspiracy theories to make the whole schmozzle hang together (e.g. you have to believe that all these thousands of highly trained experts are either all mistaken or, worse, conspiring to hide truth).
You and others may think this is not playing fair, but I suggest a major distorting factor in this whole debate is that, in the interests of politesse, we are forced to walk on eggshells as we point out the glaring problems with a worldview that has entirely been discredited except, of course, in the niche of American fundamentalism.
Now about the serpent. So what that it is described as on its belly in Isaiah? One doesn't need to be a genius to realize that the author of Genesis could have concocted a myth whereby the snake functions as a symbol for evil. And what better way to underscore the subjugation of evil in the world to come than by representing the serpent as still consigned to its belly.
But I think there is a better counter-argument: Isaiah mistakenly believed the creation account to be literal! And who could blame him - unlike we in the 21st century, good old Isaiah certainly knew nothing about evolution. So, naturally enough, he writes his material about the serpent, intending to be taken literally. But, the worldview undergirding his intent has clearly been discredited.