I agree... about a dozen seems good for your situation. Eggs are "best" when they are freshest. It sounds like you get about a dozen eggs in a weeks time. I have incubated eggs up to two weeks old with good hatch rates, but not quite as good as those under 10 days old.
There are many things you can do to increase your success rate. Here are just a few tips:
1. As you collect your eggs, store them in a cool place (50-60 degrees) and turn them a few times a day.
2. Before you set them run the incubator for a day or so to be sure the temps are stable.
3. When you go to set them candle each egg (a bright flashlight works well) and don't set any with hairline cracks.
4. If you have enough eggs to choose from try to pick average sized, normally shaped eggs.
I have kids myself, and the first time we incubated eggs I made sure to not promise more than I could deliver. I only told them we were going to "try" and made sure they knew we might not have a single chick hatch. I also was careful to warn them about the possibility of a chick starting to hatch and not making it out. I basically prepared them for the worst. We ended up with a decent hatch for beginners, and they were very excited for every chick that hatched. Becasue they were prepared, they didn't give a second thought to those eggs that didn't hatch.
Best of luck! And be sure to plan to do nothing more than sit and watch eggs hatch for a day or so!!! My kids couldn't take their eyes off of them and neither could I.