Oh, and my number one suggestion for you would be to add an Xtracycle
to your bicycle, then a PeaPodLT
This is by far the cheapest, safest, and most versatile option for you. The Xtracycle attaches to your bike, extending the wheel base and adding what they call freeloaders, which are just big pockets that you can fit anything into. Just yesterday I hauled 6 half-gallon canning jars and 50 pounds of groceries home, no problem. Add the PeaPod seat and you've got a great seat for your child until they're four years old. You can also add a second one behind the first, if you choose to.
All of those features are great, but a trailer would do the same things, right? Well, this is where the Xtracycle excels. Once your child outgrows the seat, they (and two of their friends) can sit on "snap deck", which is the skateboard looking platform. Trailers have a much lower weight limit than would allow for this many kids. As your child is learning to ride his or her own bike, the Xtracycle is helpful because you can carry that bike along with you in case your child gets tired. And once your children are fully riding their own bikes, you'll still find a million ways to use the storage and hauling capacity of the Xtracycle.
Reading back over all that, I realize I sound like an Xtracycle rep, which I wish I was. I don't even have kids yet, but I have one and it's awesome. It rides just like a normal bicycle, which I promise you is a good thing. The idea of a trike might seem more stable, but once you're actually going, a bicycle is a lot more maneuverable, especially when dealing with turns and cross-slopes. It's also nicer than a trailer, because it's always attached, I've heard a lot of people say that they don't mind that their trailer isn't attached, because then they only use it when they need it. Well, I've had so many random "oh s%*t" moments on my way home from school or whatever, when I realize I need to pick up dinner, or someone wants me to take a huge, bulky item home. I was so glad that I had that extra room available to me, even though I hadn't planned on it.
OK, end sales pitch.