6yo would play this app for hours if allowed. 9yo will 'help' and get involved too.
The app lets the kids practice number formation as well as the sums. It gradually builds up from single digits to tens, hundreds, thousands, tens of thousands. The column addition and subtraction are presented clearly, the help is well targeted..
I'm sure this app is really helping my 6yo understand numbers better.
The gameplay also involves competing goals, which are great for long-term planning (6yo 'spends' stones then remembers they were being saved for a different reason)
The graphics are fun too.