The lower levels are fairly easy to teach, although the scope and sequence is not as familiar as many other programs. There is very little review, so if you have a student who struggles, it may not be the best choice. The higher the levels go, the harder it is to switch, due to the differences in scope and sequence. By the time that you get to New Elementary Math (7th grade and up), it really requires that the parent have an extremely good grasp of math concepts.
No Answer Key For Lower Levels?
The lowest two levels of Singapore Math are fairly easy to teach and do not really require much in the way of "grading". Since you will be both teaching the concepts and working through the math exercises with your student, there is no need for a solutions manual or answer key.