Rod and Staff English is already broken down by lesson in the Student Book, so it's pretty much a matter of just doing a lesson 4 days a week. I did divide out some of the longer lessons over 2 days. You can certainly get the "Preparing..." guide if finanaces permit, and it would help you feel more comfortable, but it isn't necessary.
You're kiddos would stay together for the history cycle, if you follow this scenario (and with your descriptions of their various levels, I do think it would be a good fit). Your son may need a little time to grow into listening to the readings, but they're short and interesting enough that he will over time gain the needed listening skills for the readings.
I also think both your kiddos will really enjoy the daily science in "Bigger...".
You will want to add "Drawn into the Heart of Reading" eventually, but I would probably start out with "Bigger..." and get a pattern going and then add DITHR later in the year.
"Bigger..." does include literature study with your read-alouds in the Storytime box of the plans. So, you will be somewhat "covered" in that area until you feel comfortable adding DITHR. By the time you get to "Preparing...", you'll want DITHR in place, as "Preparing..." assumes you are using DITHR 3 days a week for your literature study component.
P.S. On the Rod and Staff English, make sure to do MOST of the lessons orally or on a white board, otherwise the amount of writing is overwhelming. At our house, we only have our kiddos write one short exercise on paper for English each day. Retention has still been very good wtih this method.
Blessings,
Carrie