LeAnna,
You may just want to move into DITHR and get accustomed to it with your child that is ready for it first. Then, after half a year (or whenever your next child is done with the Emerging Reader's Set) add your other child.

I say this because your older daughter will likely read extra books on her own for enjoyment anyway, but the DITHR program will actually guide her in what to look for as she reads and in making connections and comparisons with the Bible. So, it is the next logical step in the reading path.
When considering when to begin DITHR, I always liken it to weighing whether you would omit math for a year (or for a period of time) or whether it is a sequential progression that you want to keep going with skill-wise from year to year. Literature study is the next logical step in the reading sequence, and it makes sense to keep progressing from year to year on this path as well.
Blessings,
Carrie