sorry i missed this thread. we have a
kettler teddy child bike seat, and it's rated to 50 lbs. it's the only seat sold in the US rated to 50 lbs, all the others are rated to 40 lbs.
in terms of keeping steady, that's all relative. ask any dutch person and they'll tell you that as long as your tire doesn't get flattened by the weight, then you don't have too much weight on your bike-- the dutch live on their bikes and routinely use them to haul 100+ lbs around in addition to themselves. this is routine for them, and this is a first world, western EU country. they also have less bicycling accidents and lower bike injury rates than any other nation.
proper installation is also key. you don't want to install the seat too high up, as that will raise the center of gravity too far up.
T seats (front mount seats) are less safe than rear seats (no protection in a fall), and typically only go to up 30 lbs. the co-pilot is a popular rear mount seat, but has a 40 lb weight limit.
EDIT: not sure of the relevance of this, but i thought this would be interesting to point out-- the dutch routinely carry an entire family around on one bicycle. check out this bicycle made expressly for this purpose. it's not cheap (over $1000) but most dutch have at least one very expensive bike and typically have more bikes than people.