I recommend beginning with a Chromatic Scale beginning on D. It's easy for a beginner pianist to learn and can be played in contrary motion from the outset. Let the student work out that beginning on G sharp is just as easy!
C major is difficult for small hands, but a necessary evil and good for explaining the structure of a major scale. I would soon introduce a tonic triad. Then A minor in its natural form (saving the usual minor scales until later)
Then C flat major and C sharp major (in no particular order). Initially I would always use seven accidentals in the key-signatures. So C major would have seven naturals, G major would have six naturals and one sharp (but we haven't learned that one yet). After a while, explain that convention dictates that the naturals are not used in key-signatures to avoid cluttering the score, but they are there really.
[The general idea is that the student only learns a small number of scales and extends his knowledge by transposing the scales he knows up or down a semitone and comparing the key-signatures. I have never met anyone taught this way who has ever been confused about scales of all kinds and triads of all kinds.
So once a student learns E major, he or she will know that there are four sharps and three naturals in the key-signature (the student will have learned previously that when you begin any major scale on the 6th degree it will produce the related minor with the same key-signature). Having learned E major, the student will instantly know and understand that by transposing it down a semitone, the three naturals of E major will become flats, and the four sharps will become naturals. Playing the scales on a piano is also quite straightforward, as long as it is remembered which fingers live on which black keys in each hand.]
After learning C flat major, the student will learn easily that he now knows B major as well. A brief investigation will lead to the discovery that its key-signature has two naturals and five sharps. By transpoing down a semitone, again the naturals become flats and the sharps become naturals, and B flat major is born!
And so on ...
[I wouldn't object to teaching D major as the first major scale after D chromatic. It would be easier for a very young person or someone with small hands to play because the thumb always has more room to pass under the other fingers if the preceding note is played using a black key. However, C major is better continuity from a theoretical angle. The easiest scales are probably C flat major (B major), G flat major (F sharp major) and E major, because of the lay of the black keys under the fingers and the ease of passing the thumb. In all these scales, the same fingers play the black keys, so the finger patterns come easily. For some useful teaching pieces for beginners go to this webpage and get the 'First Ten' series: www.douglaspjones.co.uk/FreeDownloads.html]