Remember, it's been reported that KC is a CSI fan. Statistically people who enjoy scripted, prime time "who dun it" drama also enjoy true crime shows as well. For this reason, in the end, I think KC would have been a little more carefull playing hide and seek with a body. And let's face it. If she didn't have trouble figuring out, and didn't have problems TRYING to figure out what to do with the child's body, then the child wouldn't likely have been in the trunk long enough to leave decomp evidence there, which in my estimate would take 4 or more days.
Because of her taste in programming, I'd say KC knows that water disposal and weighting down a body is highly risky. When decomp gasses form the body is likely to come back up, and even if you weight it down good the body could break down and pieces could come up. You also run the risk of drought or fisherman snagging it.
Burial in her own yard? Not a chance, but I think she debated it, briefly. Burial somewhere else? And wildlife or dogs could dig the body up. Development in the area could bring the remains up at some point. Cadaver dogs would likely find it. AND burial leaves evidence...disturbed earth..fresh dirt..a mound. That would alert potential searchers to the body's location as well.
Gas cans...could she have intended to burn the body? I think that would make sense since she TOOK THE CANS with her.. but smoke and fire attract attention and that was no good either.
I think she weighed ALL of the possibilities and there were potential "problems" with all of the options. What is the LEAST likely way, besides a vat of acid, that a body will be found? THE LANDFILL. That was her "last resort."
Landfill searches are expensive. Millions of tons of decomposing trash and dead animals are sent to the landfill every single day. Millions of tons of dirt are turned over that millons of tons of decomposing material every single day. It acts as compost and the amount of bacteria in that mess could probably strip a corpse in a matter of hours. I think she put the child in a dumpster, simply because she "ran out" of options that made it unlikely she'd get caught. She figured her best chance at getting away with it was the landfill. It would take nothing short of an act of God to find that poor child in the landfill. Now take into account the child has now been missing for almost a solid three months?