"The Devil" is a rather Christianized translation of what he actually said. He was talking about the Matchi-Manito, which does literally translate as "Bad / Evil Spirit." To the Shawnee, the Matchi-Manito is also known as Meearleetheena*, "The Mean Creature." The Matchi-Manito is associated with the Horned Serpent, a rather ubiquitous figure throughout the Eastern Woodland, that lurks in the murky depths of the world below (not necessarily a singular entity, though usually their is one of the species that is more significant than the rest).
The connection between the Matchi-Manito and the Horned Serpent in Tenskawatawa's theology is hinted at by the method the Matchi-Manito used to create Euroamericans. Namely, "they grew from the scum of the great water, when it was troubled by the evil spirit." From there, they emerged from the sea as a giant crab, dripping with seaweed, and made landfall in the Boston area. As you might have guessed by that last sentence, Tenskawatawa is speaking symbolically about a vision and the statements he makes regarding this vision should not be taken as literal beliefs about the history or nature of Euroamericans. Elsewhere he spoke of Native Americans and Euroamericans having both been created by the Great Spirit (Muyaataalemeelarkwua, the Finisher), but governed by two distinct superintendent spirits and for distinct purposes.
In Tenskawatawa's theology, the Matchi-Manito was solitary spirit, capable of implanting a bit of its essence into animals and humans to work through them. He is subject to the authority of the Great Spirit and permitted to punish and perplex humanity--privileges that the Great Spirit might rescind at any time. In Tenskawatawa's vision of the afterlife, the Matchi-manito resides at the end of one of the two spirit paths. The wicked are compelled to veer off the main path into the Matchi-manito's country for punishment. The very wicked are burnt to ash. Those who have committed lesser crimes are only maimed, then set on their way, only to find yet another region inhabited by the Matchi-Manito awaiting them. There they are maimed again and released. This cycle of punishment continues for a time before, finally, they are permitted to enter the paradisaical forests of the Great Spirit. The Great Spirit itself does not make an appearance in that land, nor will it until the end of days. Tenskawatawa's theology was millenarian, believing that a day of judgment was fast approaching and that the Great Spirit would soon sweep away the Euroamericans from the North American continent or transform them into Native Americans (maybe a bit of both).
*To use C.C. Trowbridge's rather archaic spelling. His interview with Tenskawatawa is the one place I know Meearleetheena is mentioned by that name, so I'm stuck using his spelling.
Sources
This is why Wikipedia is not a good primary source. The authors do not take scholarly care. In the Wikipedia article, this phrase appears:
"He denounced Americans as children of the devil..."
The citation supporting this quote points to the original quote here:
"The Americans I did not make. They are not my [the Master of Life] children, but they are children of the Evil Spirit."
The Master of Life, and the Evil Spirit were part of Tenskwatawa's religious beliefs and have nothing to do at all with Christianity, or Lucifer, aka, The Devil. Yet, the Devil is an extremely loaded term, and its substitution of the Evil Spirit, shows we should be suspicious.
So, even though they have a nice quote, and use it as a citation, they change the context, meaning, and politics of the Wikipedia entry because of lack of skill, concern, or even on purpose.
Primary sources, and careful academic analyses are where you should go.
I use Wikipedia to get major names, sources, and other general information and search from there. I would never use it as a primary source, for history at least.