I was looking at images of Doctor Who annual covers this week and I saw that on the 1967 annual cover there is an illustration of The Doctor with a boy and girl who look younger than teenagers. I am not sure who they were supposed to be and they obviously don't resemble any companions.

I know that the TV Comic Doctor Who strips of the William Hartnell era featured two grandchildren who had never been seen or mentioned in the series (and no reference whatsoever to Susan) but I know that the TV Comic strips don't fit in with the series. I can't imagine, though, that anyone from these strips would feature on the cover of the 1967 annual.

The background of the picture features a seems to feature some kind devil bird, so I'd guess the story is probably "The Devil-Birds of Corbo", and the children must be Jack Strong and his twin sister Dot Strong.

Possibly these two?

http://tardis.wikia.com/wiki/John_and_Gillian

Definitely not those two. They were the Doctor's grandchildren in TV Comic, but the Annuals were published by a different company, who didn't have the rights to the comic's characters and so made up their own equally uncanonical companions instead. (Of course, neither company was prepared to pay the extra money for the actual TV companions.)

Handily that link I orginally put up, had extra links to other Doctor Who companions...

http://tardis.wikia.com/wiki/Ida_Mortimer
http://tardis.wikia.com/wiki/Alan_Mortimer

THose two look more like it... (I've googled the image of the annual in question)

Thanks. I didn't realise that the annuals from the early years didn't have companions from the TV series, I have the DVDs of Genesis of the Daleks and City of Death which both have annuals as special features and the two annuals (1975 and 1980) both have the TV series companions (Romana to some extent resembles Lalla Ward in the illustrations but the illustrations of Sarah Jane and Harry don't really resemble Elisabeth Sladen and Ian Marter.

I have recently read about these two characters and the book that features them, according to TARDIS Wikia, seems to wrongly suggest that The Doctor is from Earth. I don't know why the book should suggest this.

Same reason the Cushing movies did. Because Gallifrey wasn't mentioned until later. The Doctor's homeworld first appeared in The War Games in 1969, though not named until a few years later.