Let's go back to the original symptom - lameness. Lameness is like an intersection in a road that takes off in five different directions. It can lead to completely different destinations. Marek's is such a common poultry disease, it's natural to think of it first. Okay. That's one road.
Let me say right up front that in thirteen years of chicken keeping, I've learned a lot about a lot of chicken woes, but when it comes to lameness, I'm still in the dark most of the time. In fact, I have a thread going right now on my own chicken with undiagnosed lameness. Lameness has so many causes, and it's one of the most common ailments afflicting chickens, but it's about as easy to figure out as why people choose the politics they do.
Lameness can be caused by viruses, bacteria, injuries, mold, insecticides, exposure to petroleum distillates such as transmission fluid and paint thinner, poisonous plants. This past spring, I lost two chicks to a poisonous caterpillar, for gawdsakes, beginning with lameness.
I'm leaning toward a toxin exposure as the cause of your little Silkie's death. The reason for my suspicion is the very short duration between onset of symptoms and death. Viruses and bacteria and injuries would all likely have caused a much longer period of suffering before death mercifully ended it.
All chicken deaths have possible implications for the surviving chickens, and it's always wise to try to track down the cause in order to remove it so other chickens don't suffer the same fate.
What I suggest is you take this time in quarantine with your kids to comb the yard where the Silkie had been ranging prior to your discovering her under that tree. Look for anything connected to the things I listed as toxins that cause lameness. I had a young chick die suddenly at my feet, following several hens turning up lame. It took me several days of going around and around and back and forth over the yard searching for the source of a possible toxin to finally spot my log splitter leaking hydraulic fluid onto the ground where I'd last seen that chick picking up grit under it.
It had never occurred to me that the oily mess on the ground under the splitter could kill a chicken.