Have a word with your local hairdresser, showing (if possible) anything they will be wearing in their hair. When I got married I took my headdress in to the hairdressers before the big day so they could see what they were dealing with - presumably the same applies to bridesmaids.
ringlets look gorgeous on younger children but i think a more sophisticated look for the older ones i.e a vertical roll at the back with jewels running up it
a mixture of the 2 would look good
french braid? what ever you decided make sure they hold like glue, I was maid of honour at my friends wedding and she had 6 little ones after 2 hours their hair had come out and I ended up doing plaits to make them look presentable