The kitchen sink is definately a bad idea, and depending on what
your throwing down there and where it goes you could be in violation
of local sewer codes or you could hurt your septic system

The EPA publishes a pamphlet on effluent disposal, Eastman Kodak
has/had a number of booklets on the subject. Ultimately your
local public works dept should be able to advise you.  

Unless your son likes to do very strange things with the drain plumbing
I can not see any harm.

Are you sure your son is not telepathically transmitting nightmares about
'Big Hairy Things That Come Out of the Drains at Night'?

I'd be more worried about the biologicals at the bottom of the sink
trap.  But then I wouldn't be much worried about those either.

In comparison with other hazards in the house B&W chemicals are close
to the bottom of the list.  The worst of them may be 'Pyro', and
that's just boiled acorn juice.

In my family it is the fastidious side that suffers from odd
undiagnosable health complaints and allergies.

Look at:

http://www.respiratoryreviews.com/jan03/rr_jan03_hygiene.html

And make sure your son gets his full pound of dirt a day.

As your in the UK rather than our errant colonies, all the water boards are
interested in is the silver content. Actually it's the same in most parts of
the US too, I'm involved in silver recovery & photo waste discharges at
work.

The average amateur wil not cause any problems to the their local sewage
treatment works, and no-one offers a servive to collect and treat their
waste, the quantities are to small.

Yes fixer cam cause problems with a stainless steel kitchen sink but only if
not rinsed properly, it'll eat it away slowly :-)  Won't be any problem in
the U-bend tho !

Just flush woth plrnty of water. B&W chemicals are very safe.

 I can't think of any photographic chemistry where trace amounts
will be of any concern. If you will be doing Homebrew, you will
encounter a few chemicals which as powders or concentrates
need very carefull handling.
 Better to keep your son from Tuna, Albacore in particular.  Dan

I have a similar concern getting chemicals near the kitchen. I believe
everyone when they say that developer, stop, and unused fixer are no
more harmful than other chemaicals you put down the sink, but in my
house we generally avoid the sorts of chemicals that your average
American household would use. I am probably more careful than most
folks here would think necessary, but there's no harm in being
cautious.

What worked for me both in terms of this concern and the way our space
is was to build a wooden sink that sits on top of removablel legs and
stands over the toilet. I set a drain in the middle of the sink and
have an inexpensive PVC drain tube that delivers the chemicals right
into the toilet. The only inconvenience is setting up and tearing down
the sink when not in use, but that's dictated by the size of our
living space.

I keep all chemicals stored under the bathroom sink with child-proof
locks and everything else above the toilet on a shelf we just
installed. I am fortunate in having a bathroom that already has a
built-in ventillation fan and a window. I wear a respirator because
the acid stop-bath and fixer make me dizzy, but I realize that this
symptom is idiosyncratic (and that the next time I get new stop and
fixer, I'll get citric acid fixer and either make or buy the
non-smelly fixer). Finally, nitrile gloves (available in a box of 100
from B&H for about $14) are a good idea. For the relatively harmless
chemicals it keeps off the smell; for things like selenium toner they
are essential.

--Phil