We homeschool, too, and have used curriculum-based placement tests (e.g., the placement tests for Singapore Math Primary Math, All About Reading/All About Spelling), state curriculum frameworks (google your state education standards, and it will give you some idea of what your state expects students at each grade level to have mastered; you can use it as a kind of checklist, if you feel comfortable interpreting education lingo), and, well, my clinical experience.
But for little ones, it was mainly child-led. Up until you need more specific content/coursework for a high school transcript/college entry, I view the only real subjects as literacy (including reading & writing accurately and fluently) and mathematics. And writing depends on individual fine-motor development. All other science, social studies, visual, musical, and kinesthetic art topics are absorbed from experiences, reading, and interest-led special activities. For very young ones, such as yours, I would be in no hurry to do written or formal work, unless they ask for it. They will learn a great deal from playing, doing household activities with you, and conversing with you about daily life experiences. And you can do a lot of math orally, as it arises in the course of living, before they have the fine motor skills to calculate on paper.
