I'm going to link you to a great description Bret has given previously about single rider lines
here.
The single rider line is used to fill in seats in rides that have fixed seating, in even-numbers of groups. For example, Test Track has vehicles that seat six. If a party of five (or a party of three plus a party of two) rides, there will be an empty seat. To maximize capacity, a single person will be pulled from the single rider line to fill that seat. If you decide to take advantage of the generally shorter wait times available in the single rider line, your group will obviously not be riding together. Most of the time, you will end up emerging from the ride with in a few vehicles of each other. But there is always the possibility that it could take a bit longer than that.
Your suggestion to have a parent go first is a good one. I would actually make a "parent sandwich" by having the adults go first and last. That way there is someone to meet the kids at the exit AND there is someone to supervise should the child have last minute second thoughts about riding. Furthermore, you should make sure that your kids feel comfortable riding next to strangers rather than next to mom or dad.
I personally would be cautious about having kids experience an attraction for the first time in a single rider situation. The attractions that offer the single rider option are all thrill-type rides. You never know what a child's reaction will be to these, even if they've experienced others before.