Stewart created shape-shifting aliens the Zygons, who have been confirmed to return in the anniversary episode.
"The Zygons are in there with all the others - particularly the Daleks and the Cybermen," Banks Stewart - who was promoting his new crime thriller novel The Hurricane's Tail - told Digital Spy.
"[But] nobody yet knows what the plot of the 50th will be - Steven Moffat and his team in Cardiff, they are quite rightly keeping it all secret."
However, the writer explained that while he agreed to let the BBC use his creations, he feels the current Doctor Who episodes are "too adult".
"My own personal opinion is that the modern Doctor Who should be aimed more at children," he said.
"People very fondly remembered being scared by Doctor Who - now the plots seem to me a little too adult."
The Hurricane's Tail is available now, priced £9.99 in paperback and £5.99 as a Kindle ebook. For more information visit www.kaleidoscopepublishing.co.uk.
> Doctor Who 50th: Matt Smith dangles from the TARDIS in new pictures
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Watch Digital Spy's 'Geek TV' review of the latest Doctor Who episode 'Cold War' below: