If half the group does not yet have children, it will divide out rather nicely. We have done a similar thing with groups that deal with biblical or psychological issues. A key value here is that people feel they are given a choice and are allowed to make whatever choice they desire. You can continue doing what you are doing, or you can join Mary Ellen in a study of Biblical Principles of Parenting. Couples look forward to bearing children.
Parents look forward to having grandchildren.
They proudly sport pictures in their wallet or purse. Growth is normal, it is a sign of health. Every class should look forward to the day when they can give birth to other groups. Interestingly enough, in the case of small groups, it is sometimes easier to give birth to three or four groups simultaneously than it is to give birth to one. For one thing, if the group grows large enough, everyone will see that it is not a small group any longer and will thus see the need to create a new group.