THe weight is dependent on the bike and the setup.
If you have a commuter bike with a basket or paniers, I would put no morethan 5kg in the basket, 20kg in the paniers (10kg per side)...You could push that number higher with Xtracycle or if you have something like Kona Ute or Yuba Mundo or something...
My bike is officially rated for 180kg of cargo (LarryvsHarry Bullitt).
Realistically I put no more than 50kg in the cargo area, I had 80kg of tools and stuff in it too, but that was with E-assist.
I had a friend with a trike, she used it to cart her 4 kids around and it worked great (so roughly 100kg in 'kid load' )...WOrked great until her husband decided to hitch a ride with them on the rear rack one day and the rear wheel collapsed!
I think there is a reasonable balance between what the bike CAN carry and what it SHOULD carry.
You have to think about the longevity of components, braking power, terrain etc.
Here's my bike with my daughter
We used to go for 4-5 hour rides around Vancouver where I live. SHe would nap in it as we rode. Good good times....
