Regarding the idea of a cart-type carrier for BOTH kids... even if you could find one narrow enough, your five-year-old might be a little "past" riding in a cart, if you know what I mean. I carted my son around in a burley solo for two years, which was great at the time (and, incidentally, narrow enough to put your 3-year-old in behind a trailercycle or tandem, as That Guy mentioned). BUT by the time my son was four and a half he started refusing to get in the thing. Your daughter may be different, but I thought it was worth mentioning. Not only was he WAY too long in the torso for the safety harnass (this may just be a solo problem, I don't know), but he was really itching to be a part of the action. That's why I'm replacing my burley solo with a trailercycle first chance I get.