I have had four kids in my old Bell kid trailer. I took the roof off, put an extra strap around the back, and had two in the seat and two standing behind the seat. It was heavy, but it worked. Just keep in mind that it raises the center of gravity...
Agreed... 7 months old is too young. You should wait until s/he is at least 1 year old, unless you use an infant adapter.
A trail-a-bike is a good option from ~4 years old until 6 or 7 years old, depending on weight/size/strength. I have one that me and my boys have put a couple thousand miles on. Often, I've attached a trailer to the trail-a-bike to haul all 3 kids.
BTW, trail-a-bike sells a child seat adapter. If you plan on consistently riding with your children, it would be something to look into...
In your case: get a trail-a-bike for the 4.5 year old and you'll have the perfect set-up. If you want to use the trail-a-bike off road, stick with Adams, not the other brands.
Last edited by hopperja; 04-27-12 at 11:58 PM.