It's great to hear your oldest is thriving with Bigger Hearts - hooray!
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It sounds like your 7 yo places squarely in Beyond. I think I'd start Beyond with him full-speed, 4 days a week, and see how that goes. He sounds ready - hooray!
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Your oldest is taking off and thriving in Bigger, and I bet you can see him taking on more independence there each day. Your 5 yo is doing LA/math from LHFHG, and could possibly start LHTH too, just to give her gentle training in Biblical history, science/health/art/devotions, etc. topics in a fun easy to do way that takes only a bit of time. But, whether you did LHTH (along with the LA/math with her you are doing now) or not, this is an easier time to do a guide full-speed with your 7 yo.
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After your ds completes Beyond, I'd start Bigger Hearts half-speed. About that point, your oldest will be more independent doing PHFHG, your little one will be starting LHFHG, and it would be a good time to go half-speed with Bigger Hearts. I personally thoroughly enjoyed doing Bigger Hearts half-speed for awhile - we fully enjoyed every bit of it that way - and my ds was able to grow into the skills he needed so well!
This is going to be an exact placement for each of your dc, so keep in mind, you can always go back to combining within HOD knowing that it is not an exact fit but a good fit, as it would be with any combining situation with any curriculum.
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I really love exact placement for each of my dc, but I fully realize that this may not be what everyone loves.
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I hope something here has helped, and I have been praying for you each day as well.
In Christ,
Julie