One of the coolest kid seats I've seen is what this dude did for his Xtracycle.
Another I saw that looked pretty cool and was recently reviewed was the iBert safe-T-seat.
Oh, and BTW, I can confirm that when they say max weight 75 lbs or whatever, that might mean a child but is equally applicable to a pony keg of beer. Those harnesses might be good at holding kids (dunno) but also just peachy at keep your Fat Tire from rolling out, too. :-)
'91 Pickup - Imelda
'08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein