I have Bigger, LHFHG, and LHTH going right now, so I'll tell you what I do.

It might help or it might not.
Either the night before or that morning I write down a list for my DD in Bigger of things she can do independently... I go over anything that needs instructions, like doing the hands on math stuff, or walking through the science experiment or any questions in DITHOR, etc. Then I send her off with what she needs for all her independent work. Then I pick another child. Generally I try to do my LHTH child because she absolutely loves school and even with the extra workbook stuff we do with her it only takes about 30 minutes. We can get completely done with her before my Bigger gets done with her ind. work. Then I get started with my LHFHG child. Depending on what the 2yo is doing I'll either do the history/story time stuff OR I'll go over her math/handwriting/fine motor skills and let her get to work. If the 2yo is being loud and clingy we do the workbook stuff, but if she's good to go play with the 4yo we do history. If we don't get to the reading stuff then I'll wait and do it at a point when the Bigger child (8yo) can entertain the 2yo. It only takes about 30 minutes to do all the things I have to read to her, so it's not much of a break for the older one.
Ok, so all that takes about an hour total, maybe a little more some days when the 2yo is being well... 2. haha It takes about 5-10 minutes to prep my Bigger child for her ind work. So then after I have finished all of LHTH, and at least half of LHFHG (most of the time we get completely done so it's more like 1.5 hours) then I'll send them off to play while I do the other stuff for Bigger. I get the left side done except for art type stuff and then move on to the right side. Then we'll do art type things.
Basically I do'nt go back and forth from one child to another. I've tried that and it so totally does NOT work with our family right now. Too many "mommy, I need... Mommy how do I?... Mommy this mommy that" if I don't just focus on one child. It takes us about 3 hours total to get everyone's school done on a day where the 2yo is just not cooperating. Only about 2.5 hours when she is willing to play either by herself or with sisters the whole time.
So my suggestion would be to get your child doing Bigger as independent as possible with things like handwriting, copywork, math, vocabulary, notebooking, Timeline, etc. as soon as possible. Then you can figure out what length of chunks you want to split people up.
One way I have done it in the past is to do LHTH while my oldest is doing her independent work, then left side of LHFHG, left side of Bigger, right side of LHFHG and right side of Bigger. Oh, and I do not count the Finger Play (LHTH) or Rhymes in Motion (LHFHG) or Hymns (Bigger) as part of our time because everyone joins in at the end to do those things. Nor do I count the music connection because we listen to the cd's in the car all the time and I just remind them of the song that goes with the Bible verse.
Hope that helps. The basic gist of what I"m trying to say is to have larger chunks of time devoted to each child instead of switching off after each subject. Have about 30-45 minute blocks for working with each child. You can get so much more done and it's so much more fluid.