I did some shopping around but resorted back to my favorite jewelry store, James Avery, out of Texas. They have these birthstone rings that you can stack and wear multiple at a time. It's perfect... Each kid will have their own ring but they can be worn all together. Here is the one for February (Amethist):
It's a very modest ring and a much smaller stone than what Jeff had in mind that I would pick out, but I love the concept! It's also engravable! So, I plan to put "Avery 2-5-10" on the inside. I can't wait for it to come in :)