In the 50s Edinburgh we were taught the tonic so fa aged 6. My memory is that a sharp on a line equates to a t in any key and a flat equates to a fa.Working up from the t you look at the placement of do' and then apply normal egbdf or face to tell you which key the piece is in.
I never progressed to proper sight reading and the tonic so fa is very useful if you sing as it gives the intervals between the notes. It is very simple to transpose any song into a suitable key.