If you go on the stroller rental sites, they tell you the max weight and height for each model of stroller. Also, my best friend used a city mini from the time her DS was born, so it definitely works for an infant!
I have a b ready that's an inline double and I love it. I cant imagine getting through the crowds with a side by side.
My b ready will accommodate a 35lb below and
45 or maybe 50! LB above.
If I were you and my tiny 6 year old needed a stroll, you bet I'd let her catch a ride! 6 is still young and park days can be long for a little person. Or an adult!!!
Maybe if it was right at Christmas or something it would be harder but we have not had any issues taking our side by side double (city mini) to crowded places including disney. A good side by side is only a door's width anyway.
I don't think a 6 year old would easily fit or be comfortable in a b ready or city select type stroller, anyway?