I second the trailer. The kids love to ride in it (don't forget their helmets!), if they get tired they can just fall asleep, and when you take it off your bike you feel like you can ride 3x as fast

I bought a cheap one at Wal-Mart. I know there are better quality ones, but I was more concerned about the budget. It works great, and the construction is pretty good. The kids are all buckled in in a harness (the one I got has the option for one or two riders, with a max weight of 50lbs per rider), and you can see the thing from a mile away with the big flag and the loud color scheme. It's also really easy to attach and has a quick disconnect from the bike for when you want to go solo.

Really, they're not too bad weight-wise. The only complaint I have about mine is the back is solid, so when we hit a headwind it acts like a drag chute. Sure does make the muscles stronger though