About 10 years ago, I had a play group situation. I probably would have handled it much differently if I had known that as my children got older I would see these same women in pre-school, elementary school, sports activities, social gatherings (because it's a small world) etc.
Be kind, be gracious and simply call both women. (I would not e-mail...too much room for interpretation...too tempting to forward your note to anyone else who might be "interested"). Something like "Just calling because you've heard they were concerned that you had left on bad terms, let them know it was just a matter of logistics (day of meetings, cost of gas, whatever...) but be sure to thank them for including you, and if pressed explain that you've found a playgroup where your oldest child fits in a little better, but you certainly appreciate their hospitality. If anyone gets hostile, remember that judgmental people are often hiding insecurities and again gently explain that it's nothing personal, you just needed a playgroup that had more older children available for your daughter to play with.
Trust me, when they have their second child, they'll view the situation through your eyes. In the meantime, take some comfort in the fact that you are obviously missed!