180l new setup for a 9 year old

Perhaps it might be worth considering looking at this from a slightly different perspective.
My suggestion would be to take the youngster to a well-stocked Fish Shop and let him/her look at available fish and let them pick - of course, some choices might be impractical, which is where some advice would be invaluable.
But, I think that if the child has made some personal input to the fish choices he (let's assume it's a boy for the minute) could become far more involved in the tank and its well-being into the future.
Remembering back to when I was a pre-teenager (long while ago, now) I was absolutely transfixed by the way livebearer-fry would (almost by magic) keep appearing. As has been suggested previously, Guppies were something of a favourite for me back then - but they were a lot stronger strains then, even though the huge-tailed ones were very expensive fish - never seen, but often talked about.
The hobby certainly has changed since then - sadly not always for the better. Livebearers from the Far-Eastern stewponds are abysmal in quality and life-expectancy, but I digress.

Anyway, as suggested at the beginning, it might be worth involving the youngster - the children of today are the Aquarists of tomorrow.
Just my thoughts.

John