Hi Malinda,
No worries about your younger one starting in the middle. She will proceed chronologically, but simply start in the middle. I have 4 kids – 4th, 1st, pre-k, infant. We are in module 4 right now and the 1st grader is fine. A family BOC is very helpful to keep track of the characters (for mom, too). I was concerned initially, but it really is no problem. I’d suggest picking whatever module you want your 4th grader to start with and just include the younger. You could start with any of the modules you choose, though if you want the family handbook that SCM publishes you should choose 1, 2, 3 or 4.
Blessings,
Christie