Bless your heart. I hate to see people get taken advantage of, and that is exactly how this sounds.
You can very calmly and respectfully address the other moms with this situation.
Maybe instead of the person driving being the person paying, it should be split evenly amongst all the moms involved (if their children are riding).
So, if gas is $80 and there are four kids (not including yours), then each mom owes you $20. The same would go for your child riding with another mom.
This way you don't have to accuse anyone and things would be more clear as to who is paying for what.
Hope this idea helps you.
Take care.