I am a new RadWagon owner and am having the toughest time finding a child seat similar to the Thule Yepp Maxi Easy Fit that fits a child over 50lbs. My son is autistic and engages in elopement often, so we feel it would be safest to put him in a seat with restraints.  I have found a few, but they're not sold in the US. 

Anyone have any recommendations on a seat like this that can be purchased in the US?