As a home educating parent I don't see a problem with kids sitting exams early, if they are ready for it. I don't think there are many children at 9 years old who would be able to ace a Geography exam. Have you had a look at some of the past papers and the specification? It's really quite complex.
We thought long and hard about deciding which exams our Year 7 child should take, and it's based on a few factors:
1) He's a September kid, so yes he's Year 7, but if he were born just three weeks earlier he'd be in Year 8 now, so by the time he takes his exams he'll be only a few months off turning 13.
2) We opted for Biology. My daughter took it last year and generally speaking the questions are quite straight forward. There's no essay writing, it's mostly just a lot of memorisation of terminology, which experiments test for which substances, and so on. With a lot of flash cards and practice papers, it's a relatively straight forward subject. At the moment he's averaging at grade 7 on practice papers, and we still have a few months to go before the actual exams
3) For his other subject we opted for Computer Science. This kid lives and breathes programming. He's been learning Python since he was in Year 5. If there's anything that would be "his" subject, it's Comp Sci.
4) He's not doing them alone. His Year 10 brother is also doing Biology this year, and both his Year 10 brother and Year 11 sister are doing Comp Sci this year. This means he has a lot of support from them if he's stuck on anything - they all do, in fact.
5) We use tutor support. He's enrolled with HomeMade Education for Biology, and LearnTec for Comp Sci.
6) He's just generally a bright kid. I know that sounds a bit braggy, but he picks things up quickly, and wraps his head around things quite quickly, and remembers things pretty well.
However a subject like Geography is more difficult, and I wouldn't put him through it at his age. You really need to look at what's required for a subject before jumping into it.
We're going with:
Year 7: Biology and Comp Sci (for reasons stated above)
Year 8: Environmental Management (a pretty straight forward subject with no arduous essay writing) and Maths (he's already studying the Maths course, but he's studying it over 2 years)
Then 2 more in Year 9, and 2 more in Year 10. Not sure which ones yet. English will be in there somewhere, of course, as it's a pretty core subject.
I would say that Environmental Management would be better for a younger child than Geography, and it would give them some grounding before tackling Geography at a later age...
but still... nine years old is very very young to take anything really, unless you have a young Sheldon on your hands.
I'm not saying it's impossible, maybe you really do have an exceptionally gifted child, but it would be very rare to find a child that is able to get very high grades at the age of 9.
It's your choice, of course, but I would advise you to find an easier subject, and maybe start in a couple of years time rather than at the age of 9.
For Geography and any other humanities subjects I highly recommend Humanatees Tutoring, but I think even Jake (the guy who runs Humanatees) might balk a little at a 9 year old attempting it.
Whatever you decide anyway, best of luck.