One reader asked about cycling with 3 children. It is possible, but I need to work out the logistics myself before becoming an advocate, but wouldn't just an old fashioned bike seat work for a younger child? The bike seat would be attached to the same bike that is pulling the trailer.
Here is another solution for cycling with your children. Great for older kids since they get the experience of balance. I've seen older children riding across Iowa on this type of bike seat. They seem to like it and they also have a great feeling of accomplishment.