When my son was 4 he enjoyed the first game!!
He is now 5.5 and is so eager to do mathematics (Let's face it!! Play Mathematics ;)
What a great game and you'll realise how good is that investment when your child comes to you and say: "Daddy is time to collect apples!!!"
This game combines mathematics (additions & subtractions) and a basic concept of capitalism.
I.E.: In order to unlock more content the child needs to collect items that he will need to sum up and when he reached a certain number he will need to exchange them for key elements (that is when he will subtract). And then more content means more maths (and trust me, your DS and DD will strive for it!!)
The best part comes when you see your child who started to work on number bounds at school, outclassing is fellow classmates as he can do easily hundreds ;)
A proud Dad who did a great choice ;)