I'm sure this has been done before, but my eyes have gone funny searching ALL the Phil and Ted threads....does anyone know when I can start using the doubles kit? My ds is 18 weeks old, and HATES the newborn bit when awake. I end up carrying him (which is rather dangerous, as well as an annoying waste of money spent on the buggy). He's totally average weight and hieght(50th centile in the red book)-which bit would I put him in? Would it damage his back muscles to sit up in it now?