When we started HOD last year we put our then 10yo daughter, who tested at a grade 2.7 reading level (and having no LDs), into Preparing and it proved to be too much for her. She could have done the work if I read everything to her, but her reading level could not keep up to the workload demanded. After 10 units we moved her down to Bigger. We actually skipped all HOD LA for the first five months of that school year to work on her reading skills directly and indirectly through a number of things (including tubes in her ears for a year and a half of ringing in her ears, a new glasses prescription, piano lessons, and Dianne Craft's Brain Training). Bigger was border line too simple (cognitively) for her age, but was definitely where she needed to be for her to be able to read her school books independently. Around January/February we slowed down the Brain Training and started introducing the various aspects of HOD LA (reading Level 2 DITHOR books - without the workbook to start, dictation, etc.).
We were blessed that these interventions helped her to accelerate her reading learning curve and (without testing to confirm) I believe she advanced at least 1 1/2 grade levels. By the end of the year (unit 25) she was actually finding Bigger easy! This fall she jumped back into Preparing where she'd left off and is doing amazingly well, and we've even begun introducing the Extensions (slower than the suggested schedule) - a completely different student than the puddle of tears we had in units 8-10 of Preparing last fall. However, I know our DD did not have Dyslexia, so your story may be slower, so that is why I share the beginning and ending of her year in different paragraphs.
If you have more time than I did to spend with your DD (I started with 3 and switched to 4 guides last year, adding a 5th this fall) she may do fine in Preparing, but the independence suggested for them (which will also make running multiple guides possible) ramps up the next year in CTC, so just Preparing may not be enough time to help her gain the needed independence, especially since it will likely be a challenge for her. (In other words, I would suggest you look at Bigger - with or without Extensions, as she can handle them - for your DD.)
As you can tell from my signature, this daughter and her older brother are both doing "lower" guides than their younger brother, but each of them is exactly where they need to be! It is far easier this way than it was trying to combine them last fall (the older brother was combined with the younger until we switched our DD's guide, and the older brother's at the same time!).
With multiple guides, I would also strongly suggest that you place your 5yo lower rather than higher as well. It will require less of your time and also allow him to learn to read in a lower guide allowing him to (hopefully) be more independent than the guides suggest by the time he reaches Preparing as well. In other words, I suggest you consider LHTH, with possibly adding the K fine motor skills books from LHFHG and K math, too, if you wish. Starting phonics, after everyone has started and gotten a handle on their guides, is another option to consider. (My K 5yo is doing the same plan: LHTH, LHFHG fine motor skills, starting phonics between Christmas and his 6th B-day in March.)
As for your 9yo, I would suggest printing off the placement chart and highlight where you consider him to place in each subject. I would guess likely Bigger or Preparing. Either way, as a 9yo, he would not have to do the Extensions. If the two oldest place in the same guide, the Extensions (if your DD can handle the extra reading) would be one way to make it "grade level worthy" (in addition to math and the other LA subjects like grammar being at her level); even if she isn't "higher" in those subjects, the extra work means that she is doing a higher grade (if that matters to her). (We occasionally have discussions here about what "grade" the kids are in based - or should I say despite - what guide they are in.

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FWIW, I know you're hesitant about multiple guides, but if it's where they fit, skill-wise, it works MUCH better to do more guides than doing fewer and pushing them beyond what they can handle (ask me how I know!

). Although I'm officially doing 5 guides this year, it's never quite that straight forward. (My LHTH guy is speeding through 2 Days every day, but only 2-4 days/week; my Beyond guy is going slower than I'd like [about 1/2 speed?] but time is so short and we're also trying to fit in some 4-H for him; my DD in Preparing is doing well, but R&S is somewhat falling through the cracks this year; my DS in Preparing [soon CTC!] needs my most help every day [Executive Function deficits] while my younger guy in R2R is loving it and sometimes finishing a unit in just 3 school days.) All that to say that it truly is best to follow where each one fits without needing to worry about constantly pushing one or holding another back. I think combining has potential to be wonderful, but for us it just didn't work. Even with our "almost 5" guides, I'm done the push of teaching by lunch each day. Each child has 1/2 hr to 1 1/2 hrs with me each morning in a set order and the rest they do independently. It's so good to know they're each getting a wonderful education exactly where they are at. I am blessed!
Blessings to you as you decide!