I recently watched the last episode of Fringe and a little confused with the end. Why they built a time machine, instead they can use September son's power to travel to 2167 or even December's power or send both of them instead of Walter or September to 2167?
They were only able to travel through time with the help of "tech" (that back-of-the-neck appendage). There is no reason to assume that the child Observer had one of those. September's was confiscated. A more interesting question might be why the Michael (the child Observer), didn't have hair, given that September grew his back after losing his tech.
I agree with Izkata that Michael didn't inherit any powers from the tech (although that means his telepathy was innate. But that could be empathetic, even though the other Observer's telepathy was tech-aided). He was an anomaly, they wouldn't waste such tech on him.
I should also note that a regular time-machine would not have worked (so they couldn't have used December's). They needed to "reset time", in Captain Windmark's words. The way timelines work in Fringe seems to allow for some inconsistencies, so merely traveling to 2167 wouldn't change the present unless they reset time. Think of it as "forcing" the universe to "recompute". (I know it's a hard concept to get around, I don't fully understand).
Michael had no such powers. In the last season it is explained that the Observers were engineered through a process initially discovered in 2167. That process increased the activity in the parts of the brain that would make humans smarter, but at the same time decreased human's emotions. Michael was an anomaly in that he retained both the increased intelligence (without the need for tech) as well as emotional capacity.
The time machine was needed because September no longer had the tech required for time travel, and Michael never had the tech, nor the ability.
You also must take everything in Fringe with a grain of salt. Having reset time as they did, Walternate would have successfully cured Peter, resulting in Walter never crossing over in the first place, essentially cancelling out the entire show.
well, if peter almost became an observer when he implanted the tech on his neck, why not perform the same procedure with september so he could recover his abilities? and the kid not having the tech wouldn't be an issue to make journey, because in the past we saw september time travel with the son of violet sedan chair's keyboard player, who certainly did not have the tech.