Now, I'm going off what worked for me last year...
I'd start with the left page with your son in Bigger. (This seemed to give us a huge momentum push for the day and everything kind of ties in to it.

) I'd come up with some options for your daughter to do on her own during this time. Puzzles, books on tape, singing, playing with toys. I'd let her join in on the activity if there is one for the day. We also usually sang the hymn together, too.

Usually there is something for the Bigger child to keep working on at this point - art, vocabulary, copying Bible verse, notebooking.
Then, I'd take your daughter and do the left page of LHFHG while your son works on his own.
Then you could do the back and forth thing for math/handwriting/English/dictation.
Snack time, you could read LHFHG's storytime to both, or BIgger's to both.
You could do science together and let your dd join in on experiments and leave son to finish notebooking if it's a notebooking day while you do phonics with dd.
I have no idea if any of these ideas are helpful to you...but my brain's in scheduling mode as I just did mine yesterday.

I think it would be perfectly fine to have your dd sit in on part of Bigger, but you might want to make sure that she's done with her "mandatory listening" from LHFHG 1st. That way if she's done listening, it won't hurt what she needs to listen to and she can go play. She could listen to your son read his DITHOR books, too. We usually ended up doing Storytime for Bigger and DITHOR during afternoon naptime for the younger 2.
You could put play breaks for both of them in the middle of the day, too, so they get to play together.
Happy scheduling!

Kathleen