A good tool but with very poor support
We have chosen Happy numbers over other solutions because it has a pedagogical approach closely related to our philosophy and a great acceptance among students and parents, and because it is in Spanish. The big problem (and the contradiction) is that although it is in Spanish, it is not suitable for the school calendar of the countries of the southern hemisphere, and technical support takes days to respond to requests.
How I Use It
We have used Happy Numbers with the Kindergarteners at home.