Good morning! The ladies are doing a wonderful job of helping you here, and some very good thoughts have been shared! It sounds like you have referenced the placement chart, and from that you have decided that Beyond Little Hearts for His Glory is the right fit for your ds. So, you are 50% of the way there! Placement is so worth the time and prayer, as it is an entire year of each of your lives. With HOD, placement is important because HOD is concerned about making sure each student receives what is best.
One thing to consider is each guide is written specifically to meet the needs of a target age range. For PHFHG, the target age range is ages 8-10 yo. So, ideally, PHFHG is the best match for those ages. With extensions, history can be extended, and higher level LA and math choices can be chosen, but overall, the PHFHG guide is written with the ages 8-10 in mind. So, I am wondering if you could please share a little more about your 12 yo. Specifically, what level of books does she read approximately? Is she an avid reader or is reading not her thing? Is she used to reading her own history and science, or have these been read aloud to her? How much is she used to writing each day in a setting (i.e. 1-3 sentences, 5-7 sentences, 8-10 sentences, multiple paragraphs, etc.)? Does she write on notebook paper? Has she had a formal writing program and/or a formal reading program she's used? Has she had cursive? How long each day is she used to doing school? How well does she work independently? How well does she follow directions? What grammar has she done? What math has she done? I know these are a lot of questions, but they will make clearer what guide she would place best in. Just from target age ranges, for a 12 yo, the guides you'd more than likely consider would be RTR, RevtoRev, or MTMM. However, the answers to some of these questions may point to a different guide, and if so, then that would be the guide to do.
WHEW! I know I asked a lot of questions, but when we are trying to help with placement advice, we are shooting in the dark without quite a bit of clear information. Especially by 12 yo, dc's skills and abilities and needs can vary greatly! More information helps us give better advice, and then after all that, the final decision is yours - the mom who knows your dd the best. So, if you could please share about your 12 yo with these questions in mind, that would truly help us help you more.

In the meantime, here is a link to some wonderful posts where Carrie has some excellent thoughts in regard to some of the questions you are pondering, like:
What are we considering, when you ask us which guide your family should use?
Why do we ask you to look at the placement chart for each of your individual children when considering the best placement for your family?
Why can’t I just place my children in HOD where they would fit next in the history cycle?
Why don’t you recommend having children younger than the target age range of the HOD guide simply listen in with the older student’s guide?
What would my child be missing out on if I did choose to combine him/her in a guide that doesn’t fit him/her well on the placement chart?
How will we be learning as a family, if we do separate guides?
How can I get my different-aged children into the same guide, or how do I know if I should try to combine them?
Within the link below, you can click on any of the above questions to read Carrie's responses, which I have found to be so true with our dc, 2 of which are exactly the same ages as your precious duo (8 yo and 12 yo; and then we also have an almost 16 yo). HOD is worth this time spent. 12 years of using it and loving it with our own dc is my testimony. I want that for you too.
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In Christ,
Julie