I start teaching scales, chords and arpeggios as soon as possible.
An approach that works really well for learning scales (whether 1 or 2 octave), which I have used with 8 of my private students, and 2 students at the school I am a music teaching assistant at, is the following, which the numbers refer to scale degrees. Each is to be repeated at least 5 times:
Play 1-2-1.
Play 1-2-3-2-1.
Play 1-2-3-4-3-2-1
Play 1-2-3-4-5-4-3-2-1
Play 1-2-3-4-5-6-5-4-3-2-1
Play 1-2-3-4-5-6-7-6-5-4-3-2-1
Play 1-2-3-4-5-6-7-8-7-6-5-4-3-2-1
For two octave scales, have each octave done separately, then combine them. Two of my students commented on this approach, which I happened to use in my own scale practicing on the piano one afternoon, made scales really easy (one who said "so easy it's ridiculous" (he is a clarinet student, while he was learning Ab major (not that easy on the clarinet), and another (a piano student) who used to find scales rather difficult, and now plays them quite fluently since trying this approach. (and who, lately, has been playing with a lot more confidence!)
Seems to do wonders for developing a sense of easily crossing the thumb under, precise muscle memory (how many piano students do you know mess up the 3-1 and 4-1 crossings in scales?), and getting used to the tone + semitone gap in the harmonic minor!
Meri