When people ask me about homeschooling, I almost always get the question about what type of curriculum is best. When I started out, my husband and I mainly concentrated on the Arabic. Around age four the kids recognized the letters, and then began to trace them, and then by five they could read and write simple words. At age 5, I did not buy any curriculum in English. I just taught them myself to read and write and do simple Math. Once my son turned 6 and my daughter 5, I had heard of the K12 http://k12.com. It was a pilot program starting up in my state and I decided to try it. It is actually public school from home. They provide the books, computer ,printer, and teacher assistance. They even pay $12.00/month for your internet connection. Well did I mention that this is all free because it is public school. It is a good curriculum and has very intense learning objectives. This is the positives. The downside for me is the time constraint to finish the curriculum. It is public school. Therefore ,you have nine months to complete the year. I do not like to feel rushed and sometimes this curriculum makes me feel this way. Also, you have to participate in the state standard testing. I hate these tests. I do not feel they reflect the true learning degree of the child.
As far as the Arabic, my husband continues to give the children a full lessons of Quran, History, reading/writing and Hadith. These studies are all done in Arabic. May Allah bless my husband's efforts. I have two younger daughters one is three and the other 15 months. With my three year old I have been giving her Arabic Alphabet material from this site: http://yemenlinks.com . This site is very nice and the material is great for the little ones starting out to learn Arabic. May Allah bless the efforts of the sister who developed it.
In closing, I think buying a curriculum is a risky task. You never really know how it is going to be unless you try it. While considering what to do, I would research and take recommendations of those who are already have tried certain curriculums. This way, you have seen actual experiences to go by