I understand the situation you are in. I also have some time constraints when it comes to teaching. I don't own a store or teach at a local college, but my career requires me to be ready to go 24 hours a day and 7 days a week. When I began my children's classes I decided that children under the age of ten were too young (basically I felt that children younger than ten did not have adequate motor skills to learn Aikido), I was wrong. I now teach basic Aikido skills to children as young as six. I've found that the children are able to learn the basic movements just as the adults and in some cases, even better. I've found that patience is the key and I expect the children to learn the same techniques as the adults with respect to kyu rank. Do I have formal testing with the children? No, I do not. I pay very close attention to each child (I have small classes) and when I believe that each child is capable of performing the techniques properly (including Ki exercises such as Udefori Undo, Tenkan Undo, Kokyudosa, etc.) I promote them. Some people may find this method of promotion to be a bunch of garbage, but I do not. The children come to class every day thinking that today could be the day they are promoted and because of this they work much harder.
In my adult classes I do have formal testing. Adults for one reason or another feel more at ease with the testing environment. You can't hold the children to the same mental and physical standards as the adults, however you can expect them to be able to properly demonstrate basic Aikido principles and movement. For example, I've found that the children are much better at rolling and falling than the adults. It seems that the adults are afraid of smashing their heads and the children are not.
You have stated that when the children begin to mature physically (they get bigger) you move them into the adult classes. I do the same although it is usually after they turn twelve or thirteen.
When I first began teaching the children, the parents were in the same area and could observe. I found that the children didn't pay as much attention to me as they should and the parents often added their "two cents". An example would be, "No Joey, you need to put your left foot forward, no Joey the other foot", says a parent. The parents can be distractors. Now the parents use the fitness centers exercise machines and running track (good for them) while I teach the classes.
I hope this painfully long post will give you some new ideas on your childrens classes. You have stated that this is your first year in 20 being away from childrens classes. I've probably not written anything that you haven't thought of before, but you asked. Good luck!