ok, I guess I'll take the unpopular stance.
For safety reasons I'd rather see kids sit and adults stand. Not all children are small enough to comfortably sit on people's laps (7 and up), but neither are they tall enough to reach the overhead straps/bar to hang onto. The buses often make rather sharp turns. In some spots on the bus there is no vertical handrail to hold onto, particularly if you are near a wheelchair accessible area. If the bus made a left or went fast around a curve a child could be pitched to the side. It's hard enough for adults to hang on sometimes let alone somebody who can't get a good grip.
Frequently on buses, I gave my seat to a kid who was too big to sit on somebody's lap, or those laps were already full. A healthy adult can stand just fine, but I didn't think it was reasonable for somebody who would have a hard time holding on to have to stand. To me that would include anyone elderly, pregnant or under about 5 foot tall. I would give up my seat for somebody who looks like they need it, but for reasons of safety I wouldn't make my 6 year old son. (who is 4"4 & 70 pounds and not sitting on my lap. lol)