No time for a very long review. Sakura Quick Math is free today (normally $1.99) This is a good mental math drilling app for kids (and adults if you are a bit rusty). It covers addition, subtraction, multiplication and division. There aren’t a ton of bells and whistles, but it does have varying levels of difficulty. It also tracks your performance against your past response times as well. The only drawback of this game is that it lacks a keyboard or typing interface and relies on handwriting recognition. This means that occasionally the app may recognize your written numbers incorrectly.
I would get this since it is free, but there are other math apps out there for the same price that are more engaging.
Recommended for ages 6 and up. This can be used with younger math adept children who have mastered writing numbers.