The best way to do this is to add something to the beginning of the kids' names in each group so the Pensieve will automatically sort them. For example, if you put a '1' in front of all the kids in your first group and '2' in front of the kids in your second group, all the 1's show up first and the 2's following. You could separate them even further by labeling them 1a, 1b, 1c, etc.
If you've already put in names, you can go back to each child's names, edit and add the code you want--you'll only have to do that once.
Hope this helps!!
Suzanne