DO NOT PUT LETTER STICKERS ON KEYS!
I was also about to say Don't consider Dogs and Birds, as well but it looks like I am too late!
I taught 'informal ' music in primary schools for twenty-five years - keyboard, recorder, percussion - and the gimmicky things you are proposing makes it HARDER for a child to learn!
There are only EIGHT white notes (including C and an octave higher C) and, including thumbs, you have TEN fingers. In the first 'baby' music, each finger in the right hand has its own note, so we are only looking at FIVE different different notes.
Unfortunately, many people seem to consider music is somehow too difficult for ordinary mortals to master, and it become steeped in elitist mystique!
Dogs and Birds introduces an EXTRA step the child has to learn, namely 'what letter goes with what creature'. This then has to be translated to the keyboard, an extra and unnecessary step.
Our DS started on our Yamaha organ at under two, and was reading music, making up tunes and singing along to them by four or five.
If I haven't annoyed you too much already, can you tell me what sort of keyboard it is. Are they full size keys, and ideally there should be 61 of them? Or is it more of a 'toy' keyboard?
[Sometime, I can talk you through how to teach him, should you be interested.]