M.B.
I loved my Burley when my kids were younger. It was expensive, but well worth it. They have both a single and double version. I have the double one and I used to strap the infant seat into the back. Once the kids were out of the infant seat, I'd strap them back there with their bike helmets on (they need to learn early about using the helmets and Mom and Dad should also wear helmets as an example). The double burley can hold one child up to 75 pounds or two up to 100 pounds and I used it to it's limits. It's also convertible and can be used as a stroller. We never needed a regular stoller after we got the Burley (my kids were 2.5 years apart).
Not totally related, but I would not recommend later buying the single wheel burley that you can also hook onto the back of the bike for the younger/smaller kid as you won't get alot of use out of that.