That sounds complicated and expensive, I think there will instead be a number of preferred cars in service, some for 1 baby, some for 1 small kid, some for 1 baby and 1 small kid, some for 2 small kids... or simpler, family cars with 2 variable kids‘ seats installed.
more likely a standard attachment system - like today isofix but with more payload options - with people purchasing (or renting as part of the service itself) the options as needed
