I have been in a carpool with another mom for nearly three years. About a year ago we added a child to the carpool whose family doesn't help in the carpool and wants to add another child to the carpool. Our vehicles don't have additional room so I assumed that the new family would drop out. Instead the other moms organized an alternate plan (without my knowledge) that will not work for me. I told the original mom and she didnt seem to care that the new plan meant me leaving the carpool.
No good deed goes unpunished, right? The world of parenting is just like the rest of the world -- not everyone is thoughtful and considerate. All you can do is calmly state your suggestions. Perhaps try something like this: "I'm sorry but the original carpool just doesn't have room for an additional child." If the other moms decide to work out their own carpool without you, that will be a disappointment -- but perhaps it's just as well for your child not to be in that situation. Next time, you'll be able to draw on this experience to make sure everyone has a fair share of responsibility -- from the beginning.