You happened to catch me on the board, so I'll chime to answer your post with my thoughts first. I'm sure other moms will be along to add their wise 2 cents too.
I know moms who have done Beyond with a 5 and 1/2 year old. My own sister is doing that very thing this year. She is underway with Beyond with her 5 and 1/2 year old right now, and he will turn 6 in March. He's doing just fine and enjoying school.
If your 5 year old is able to listen to the readings in LHFHG, then she'll be able to listen to the ones in Beyond as well. You'll most likely want to pick either the Boy or Girl Interest Set (or a combination) to read-aloud, rather than the classic books, as the classic set requires a more seasoned listener.
I also wouldn't start the spelling words in Beyond with your daughter, until she is at least 9-18 weeks into learning to read. So, you'd just wait on the spelling lists for her until then and start them mid-year.
As far as the handwriting goes, I wouldn't worry about rushing her there either. You can easily have her do "A Reason for Handwriting 'A' in place of the copywork in Beyond to make that area better fit your daughter. The rest of Beyond she should be fine doing as is.
One other option you may want to consider is doing Beyond as a 4-day plan to allow your daughter time to grow up while you're doing Beyond. Then, you'd just go 9 weeks into the next school year to finish Beyond, and she'd be right on track age-wise to start Bigger right along with your son.
As a back-up plan, you could always downshift into doing Beyond at half-speed if needed and then bump up to full-speed when your daughter seems ready.
All of these options will allow you to keep your 2 kiddos together, which will work great for you in the long-run. It will also allow your kiddos to get the added benefit from the HOD guides building one upon another.
Blessings,
Carrie