The 106 Days of Creation uses “Considering God’s Creation” (which goes up to 7th grade) for the older children. So you could do that with all of them later.
Or you could cover similar topics and just use different books or have the older one or two read extra on the topic.
Or you could have the oldest one or two learn about something and “teach” it to the youngers. We’re going to try this this year. My oldest (12) wants to focus on birds and my second (11) on insects/spiders. So they will be teaching my younger set (ages 8, almost 7, and 4) what they learn. I will probably re-use this idea many times as my next ones get older and my 2 year old and 3 month old start joining us.
I do agree with CindyS about the nature study being the most important and the easiest to adapt to multiple ages!
HTH, Rebecca