Hi L.,
I know some parents always feel there is an age limit to when you should ride with the kids, I on the other hand didn't feel that way. As soon as I was able to get the OK from my doc, I was back on my bike (about 8 weeks post c-section both times.) I learned from having my first that the bike trailers are GREAT. I REFUSE to use or suggest ANYONE use a co-piolet chair on their bike. If something should happen to you or the bike and the bike goes down, you can jump off, the baby can't. That's just a risk I am not willing to take.
With the trailer though, they make them double occpancy, and the one I got was big enough to fit my 2 year old and the baby's car seat. I just strapped in the car seat with the harnest that my oldest used just like I would have strapped the car seat into the car. Of course I was careful on bumpy areas and things of that nature. while they both were really little (not able to control their head yet), I also used one of those padded head surrounding things in the seat as well to protect their heads from bopping around during the ride.
It's great to get back out there and have fun riding again. Don't let the kids age stop you, just make arrangements to bring them along.
ALSO, I know this is only one mom's oppinion, but PLEASE DO NOT trust your child to a co-piolet seat. YES they may seem the cheaper and easiest way to go, but they are also the most dangerious if for some reason you need to get out of dangers way. At least with a trailer, they are large enough for drivers and others to see, they have plenty of room for the children, and most of all, if something happens to you on the bike (it tips over, busts a wheel, etc.) the trailer will stay upright even if the bike is laying flat on it's side!!