Safe child seat attachment 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.