You choose chimp to contrast human because chimp has the most similar genome to human.
Not really, because "similar" could mean all sorts of things. We choose the one that is most closely related, because their genome and ours used to be identical.
Pay attention here: It is NOT because chimp and human have any evolutional relationship (common ancestor). So. this is NOT an application of evolution principle. It is simply a simple logic. We do not need to check DNA to see chimp is an animal most similar to human. A child can see that.
You're assuming that similar animals have similar genomes. Why? There's nothing in creationism that requires that. In fact, genes doing identical things can be quite different at the DNA level. The only reason we originally sequenced the chimpanzee genome is because evolution told us their genomes should be very similar.
In fact, your criterion for choosing a comparison animal -- similarity that a child can see -- can go badly wrong. A child can see that the closest animal to this one
is something like this one:
Certainly a lot closer than something like this:
But if you follow that intuition, you'll be wrong. The whale is in fact a much better choice for a matching genome. That's because it's more closely related to the first animal, despite being very dissimilar. (Mind you, I've never actually compared these genomes. Consider this a prediction of evolution.)
However, it is true that, even not knowing any evolutionary biology, once you actually compared a bunch of genomes you'll discover that that chimpanzee genome is generally very similar to the human one. Does that tell you which variant is ancestral? Well, that depends. Where does most human genetic variation come from? In the creationist model, is it from differences among the chromosomes of Adam and Eve, or from subsequent mutation? It's impossible to say, since there is no creationist model of human genetics. I'm really not sure what to expect if evolution isn't true.
Nevertheless, it was just to be safe that I included my second application of common descent -- estimating the range over which mutation rates vary. Because that one makes no sense under a creationist model at all, whatever assumptions you make about where human variation comes from. I'm careful that way.
(First photo: North Carolina Wildlife Resources Commission, 2nd: SunCoast Sugar Gliders. Both CC license.)