I can't read music neither can DH, but our two boys are very keen on leaning instruments. DS2 started piano lessons before DS1, about a year ago, and has passed his prep test and is learning grade 1. DS1 is doing prep this Friday. They can read music ok, both clef, but not sight reading as such. They still have to work some of the notes out. They are 8 and 9 years old.
My MIL is quite pushy that by now they should know all their notes and where they are on the keyboard, I am more of the opinion that they are progressing well and with confidence, and the sight reading will progress as they learn. So from your experience, how long does it take for children to integrate all the notes and 'know' them completely?