I am debating what seems best to me. It sounds like from what you've shared that if you were to place them individually, you would place the youngest in Bigger, the middle in Preparing, and the oldest in CTC, right? I keep going back and forth in my mind as which way to go. I'm thinking of 3 possible scenarios.
First, you could combine the older two in Preparing having the oldest do extensions. This would probably work really well for both of them. Then you could put your youngest in Bigger where he places best. This would probably be great placements for all three of them, but the most teacher time. Bigger is probably the most teacher intensive guide since most of the work is done with the teacher. The history, science, Bible study, poetry, storytime are all read by the teacher and most of the activities are teacher guided as well. This is the year that you work side by side with the student helping them gain the skills that they need to be independent in future guides. It will probably take about 2 to 2 1/2 hours of teacher time leaving only 1/2 to 1 hour of independent time for the student. As the year progresses and everyone settles in to the routine, there will be more and more that the student can do independently. It's possible since your ds is a great reader and already 8, that he would be able to take over reading his science or history himself fairly early on which would help free-up some of your time. Preparing is more independent with the students taking over the science box, having an independent history study box, and quite a few of the activities can be completed independent of the teacher. The teacher time with that guide is more like 1 to 1 1/2 hours each day with the other 2 to 2 1/2 hours being independent.
Second, you could combine your older 2 in Preparing as before but have your younger ds in Beyond. This would be less time intensive for you since Beyond is overall just less time per day. I'm thinking though that Beyond might be too little for your advanced 2nd grader. We did use it as a 2nd grade program using the appropriate levels of math and LA for second grade and my dc loved it. It didn't seem too light and we had really great days so it could work for you too. One of the benefits of choosing this path is that it would allow you to focus more on getting your older two up and running and more independent so that the following year, you could really focus on moving your younger son into independence. This would also give you a year between guides so you wouldn't be doing the same guide two years in a row.
Third, you could combine your 2 younger dc in Bigger having your dd do the extensions. Then you could have your older son do CTC. This might work though I wonder if your dd wouldn't feel challenged enough in Bigger and I wonder how your older ds would be starting in CTC. It's really a meaty guide with quite a bit of reading and writing that is done independently. Some dc have problems with the reading level of the history text. I think it is the first one, Story of the Ancient World, that some have difficulties with just because it is an older style book. My dc haven't had a problem with it, but they both read above grade level and they had already done Preparing before this year. So they already had an understanding of the flow of the history and had mental pegs to hang the information on. I think that has really helped them this year as they study some of the same topics but from a different perspective or more in depth.
Those are some of the combining scenarios that come to my mind. I'm not for sure what would be best for you. Perhaps if you look at the first week of each of the guides that will help. In the guides from Preparing on up, the boxes are listed with T, S, or I in the corner of the box. T indicates this is a box that will require you to teach, so pretty much plan on that box being done all together with you. S means that box is semi-independent. That means that after you've read the instructions together or in the case of grammar or math done the textbook portion, they can do the rest of the activity themselves. So usually about half of the time or less of that box is spent with you. The I boxes, the student does independently. At the first of the year, you may need to answer a question here or there, but for the most part that box is theirs and they will do it without your help. Looking through the first week of plans for each guide to see what would be required of you and what would be required of your dc might help you figure out what would be the best fit for your family. If you need to see the approximate times that each guide takes, it is in the schedule thread on page 10:
viewtopic.php?f=6&t=2093&start=135 The other thing to ask yourself is how you feel about teaching the same guide 2 years in a row. Some people don't mind, others do. If it's something that bothers you, I wouldn't do option 1. Hopefully some of these thoughts will help you ponder your options more. In the meantime, I'm sure more ladies can offer their ideas as well.
I don't think it would be a good fit, without lots of tweaking, for your younger guy. He could listen in on history, storytime and bible. Science is scheduled as independant, so you could have the olders do science on their own, and then find another science for him.