It looks like a wonderful idea - but my goodness , it's expensive. (Focussing on the good things that happen in life, no matter how tiny, is a proven psychological and spiritual way of raising happiness/ gratitude levels.)
I wonder if there's a way of making something similar cheaply available for children whose parent haven't got 20 quid to spend on all of their kids' Xmas presents, never mind one.
There are a number of adult equivalents eg The Gratitude Journal (I have used one in the past - when there was only one - it seems that there are several now, depending on how spirit;/ religious/ atheist you prefer your happiness).
Lovely to hear that this technique is proving useful for children though - it will give them a lifetime habit of optimism, without being unrealistic about things, which has to be good*
Don't worry - your child will not become that nauseating little squirt Pollyanna* 