I was told by my bike shop that the bike I have can only mount a child seat at the seatpost. My son is 2 and I need a safe child seat for him. I have been having great difficulty finding one that mounts at the seat post online and I am reading that its not the safest if not mounted on the back as well. Any suggestions?
The conclusions you've drawn from your research are correct. There are racks that are specifically rated for child seats, and they attach to the bike's frame into a pair of rack stays near the seatpost as well as near the rear axle. Any rack rated for touring or hauling heavy loads reliably will use these attachment points at minimum.
A child seat should be mounted on a rack that attaches firmly to the frame of the bike - which usually includes the seat stays near the seatpost. There are racks that attach onto the seatpost themselves, but these are lightweight racks, generally meant for bikes that need a rack only occasionally, or for road bikes without rack braze-ons; such "seatpost racks" are designed for easy removal and attachment, and are not designed to hold more than the weight of a daypack. Mounting a child seat on one of these would be horribly dangerous.
I sincerely hope that you've misunderstood your shop. Attaching a child seat to a seatpost rack would probably break the rack and then harm the child. I suggest avoiding any shop that gave this advice.
But I find it difficult to imagine that even a marginally competent LBS employee would recommend this. Is it possible that what the shop said was to get a rack that attached to the frame near the seatpost, as well as near the rear axle?
Does your bike have attachment points in these locations? from your question, I'm guessing it doesn't, and I'd seriously reconsider and use a trailer instead of a child seat with your bike.
There are also racks, (good quality, safe racks like this one) which only attach to the seat tube. NOT the seat post, but the seat tube on the frame.
They are strong, rated to 45kg, and fit most bikes.
After a few days with no accepted answer I figure I might throw something in that is a little outside the box:
I used a Kelty Child Carrier for my kids until they were old enough to ride their own bikes. Something very similar to this: http://bit.ly/JhWk7J
I used it to go downhilling, trails, actual hiking, skateparks (bowls and some smaller jumps), store, school (I was in school and take them sometimes) and when you get to where you need to dismount, you can leave your bike and keep the kid strapped, VERY useful after they have fallen asleep, then when you get to your destination they have a chair right there and you never had to wake them up for ANYTHING! I modified it with a camelback chest strap to keep the child's shoulder straps together so they wouldn't fall out if slightly inverted. The best part of it was the waist belt, get that strapped right and you dont even notice the kid on your back!! So nice! I gave mine to my brother when he had his first, and he LOVES it too!!
I bike with two children, now aged 1 and 4, and have tried a number of solutions. A couple specific suggestions for you:
First, some child seats come bundled with their own rack. One of them is the Topeak BabySeat.
Second, heavy duty racks include weight ratings. Old Man Mountain is a respected brand and has a rack in their line rated to carry 70 lbs. Their racks are here. (Beware, it's not cheap!).
My personal experience has been with carrying my kids on a bakfiets, Xtracycle and an electric Yuba Mundo. If you are interested, I have a lot more information and photos about those options on bikes-as-transportation.com. Good luck!