So, the first stop would be the placement chart (
http://www.heartofdakota.com/placing-your-child.php). Once you determine what guide each child places into, then you can go to that guide's page. Based on age alone, one or both of your kids might place into Bigger, so I will use that one as an example. Here is the Bigger website:
http://www.heartofdakota.com/bigger-hearts.php. At the top of the page, you will see the link to Bigger Hearts Packages. This is where you would go to see what items you need to purchase. A little below that you can see the links to the first week, the introduction, and At a Glance. I highly recommend looking at these before going to the packages page to see if you think HOD will be a good fit or not. HOD is complete with options for math and language arts. But, it does allow a family to use what they are most comfortable with as well as what they suggest. On the packages page, you should see an economy package for history. This package will include the guide (with daily lesson plans so there is basically no prep work), all of the history books, and sometimes other subjects that only need one book. Then, there are packages that include science, read-alouds (which expand the history), math, and language arts. Depending on the Bible study planned in the guide, there might be books to purchase for that as well. The extensions should only be used if a student places into a higher guide but is using a lower guide for any reason. I would say history, science, math, and language arts are necessary. Bible and read-alouds are very beneficial, but if money is tight, you can skip them.
I hope that helps some. If the placement chart isn't clear for your kids, you can post about what they are reading, how well they write (copy work, handwriting, etc), and others can help with placement.