It depends a lot on the individual CM's. The rules say that all kids under 7 have to ride with an adult. Technically, they would not let your 6 year old ride with your 7 year old and this can be a big pain on Buzz Lightyear, Peter Pan, Astro Orbiter, etc.
How old does your six year old look? Could she pass for seven? I would just tell your two older ones to pretend like they know what they are doing and just board together.
Once at Dumbo, my kids looked so crushed that a nice cast member rode with one of them-on Astro Orbiter I asked a stranger to. My kids were only 5, 5, and 4. We had a great trip though-I just think you need to be prepared.