Cyndie, I'm glad I could help both of you!
Start with what they know and go from there. You may want to get a more structured curriculum for math and writing, and unschool the rest of the subjects. Younger children develop at different rates, so it's hard for me to think of any child the age of 6 as "behind." I encourage you to stop thinking that way, too, regardless of what the school may have told you.
For your youngest, try starfall.com for learning how to read. For your oldest, let him/her read as much and as widely as possible, tying some of his/her books into science and social studies topics.
For math, consider Miquon Math, Math-U-See, and Singapore. My boys used a combination of Miquon and Singapore math programs, but my daughter does better with Math-U-See. There are other resources if these don't appeal to you.
Here are links to all three so you can read more about them:
There are a lot of resources for writing. Here are two:
I use a variety of resources to mix and match, so hopefully others will jump in with their favorites.
For handwriting, try http://www.handwritingsuccess.com/ (but only for print - don't use the cursive)
For science and social studies - coordinate this with their reading as much as possible, and go to museums and zoos, take field trips, visit historical sites, watch educational TV and documentaries, do experiments, etc.
I also love the Cricket magazine group magazines: http://www.cricketma...ist.aspx?type=M and use them extensively in our homeschooling.
Call them to ask for samples (not sure if they still mail them out free or not). Try Click for your two youngest. Your 9-year-old may still enjoy it, too, but may enjoy Ask better (those relate to science and social studies). Try Appleseeds for your oldest (it's history/social studies, mostly, with science from time to time). The age guidelines at the website skew high, in my opinion. (Muse, Odyssey, and Cobblestone, should wait for after children outgrow the younger versions.)
I like their literary magazines, too (try Ladybug for your youngest, and Spider for your 9-year-old).
I guess I've thrown enough at you to start with. Let me know if you have any questions. Others will probably jump in with their favorite resources, too!