It can be difficult to determine which 'grade level' a child is at for maths because they can be strong in one aspect of maths and find another aspect very challenging. It is more useful to ask which skills they know and which skills they need to work on next. For example, if your child can add and subtract single-digit numbers, then you might want to work towards adding and subtracting 2-digit numbers and/or beginning to learn times tables.
The descriptions for each Maths Core can then help you to determine a starting level. The SAP Learning Maths books are quite cheap, and these can be used to establish skill levels and challenges. For older students, you might use Khan Academy (which is free and online) to determine their skill level. Another alternative is to begin with All About Numbers, as this Core is not sequential maths and will give you time to understand what your child knows and still needs to learn. If your child is struggling with basic addition/subtraction/multiplication and division, we recommend beginning the 2-minute Maths PDF book, which is available in our Resource Shop.