I think you've done everything right so far. I am a blended family as well and I have the same scenerio in the summer. One week with all 4 and one week with just 2. My blended family get's along like their best friends rather then brothers and sisters. So, you may have a perfect arragement.

I would certainly take 2 weeks though. It's going to show intent to stay through out the summer and it will show that they have the income to pay.