playschool,
Sorry it's taken me a bit to get back to you! The ladies have been great about talking through your options with you!
I agree that Bigger Hearts is the best overall fit for both of your older kiddos, along with the extension pakcage. Both kiddos would do the left side of the plans in the Bigger guide, along with the storytime and science on the right side of the guide. Your younger one would do all of Bigger as written. For your older child, I also agree that you'd need a higher level of math, english, literature, dictation, and additional science beyond what is in Bigger.
The literature is easily solved by adding Drawn into the Heart of Reading Level 4/5. The extension pack books from Bigger won't work with DITHR (since they don't cover the various genres), so you'll need to choose your own books for that. We do have packages for Level 4/5 coming out after the first of the year, so you could always peek at those to give you some good ideas. You could use the package titles on the list as a help of what to get at your library to use with DITHR, or just order the books from our packages to have on hand. Any book truly works with DITHR.
As far as math goes, it sounds like you will be doing your own thing with that which will "up" the level of math. For English, depending on what your daughter has already had in her English study, you could most likely use Rod and Staff 4 as a good starting point (or Rod and Staff 5, if she's already had quite a bit of diagramming and overall grammar instruction). The goal with HOD is to go through Rod and Staff 6 by the time kiddos get through grade 8. This is due to Rod and Staff being so advanced overall.
While Rod and Staff does include some writing instruction, you would most likely want to add a higher level of writing by continuing with your Writing Strands (or by doing Igniting Your Writing Vol. 1, which is on our website).
For spelling/dictation, you can do your own thing, or you could use the passages found in the Appendix of Preparing Hearts. There are also posts on the board here on how to do dictation from the literature you're using for DITHR, rather than using the prepared passages found in Preparing Hearts. If you did choose to use the passages in Preparing Hearts, you could also make use of the Rod and Staff English 4 schedule and possibly consider following the science schedule found in Preparing (instead of the one in Bigger) to "up" the level of science for your older child. In that scenario, your younger one would still do the science in Bigger. Just something to ponder.
The science scheduled in the Extension Pack in Bigger will help add on higher level science content, but since your daughter will be a sixth grader, you'd most likely want to add additional science reading to Bigger either from library books or an elementary Apologia text once or twice a week for the year (or you could have her do the Preparing Hearts science instead of the Bigger science and that would cover science for her).
As far as your younger child goes, Rod and Staff 2 or 3 would work. English 3 begins diagramming, so if your child is ready for that, 3 would be a good fit.
I'd love to hear what you're thinking if you get a chance to pop back and respond!
Blessings,
Carrie