As someone who's followed Adam Buxton's career for at least a decade - I can say it's not fake (and was indeed used on Armando Ianucci's Time Trumpet - possibly where the visual effects came from). I agree with FastFrets comment above, it's setup in a way (the kids playing a game with his dad to recite whats said, I think anyway).

Viral! Come on JoeDirt, are you obsessed with uncovering viral ads?
Adam Buxton's a comedian, he's obviously set this up to get a laugh. Jimmy Carr is also a comedian who fronts just about every show possible on Channel 4 - he's a regular victim of piss-taking on tv.
There's no way on earth this is a viral.

I love this clip even more now I know it's actually the kid talking...

"Tiny TV Boss was something I made last year after buying a video baby monitor to keep an eye on our son Frank. Like the Pope’s funeral bit, it ended up being used as part of my Edinburgh show and I thought I might use it somewhere else but the moment seems to have passed so I decided to release it into the netsphere. Several people who’ve seen it have assumed that it’s me doing a baby impression but I assure you my vocal skills don’t stretch quite that far and the voice you hear is genuinely that of Frank who was about 2 and a half at the time. He was genuinely talking into a phone and it’s me on the other end so it’s more or less as real as you can get. I did a few more bits with Frank along the same lines, which I may try to post in due course. I also gave Armando a thing I shot with him for possible Time Trumpet use so we’ll see if Frank makes it to the small screen when the show comes out. Incidentally I negotiated a very generous buy out with Frank for the use of this stuff, so there’s no need to worry about him being exploited. Let’s just say he’s now got more Bionicles than any tiny human could possibly need, want or destroy."

Link - http://adam-buxton.co.uk/ad/2006/07/05/you-tube-news/