We do have moms using Beyond with 7 and 9 yo dc and loving it!

Thank you for describing each of your dc so well, as that helps so much in proper placement. I think they could both do all of Beyond Little Hearts (with the exception of your subbing in Sequential Spelling for the spelling in the LA box), and you can just add some science to extend for your ds, as well as doing his appropriate level of Singapore, reading, and possibly grammar.
Moms extend the science in various ways, but one of the favorites is to have dc do independent research on the science topic they're already learning about, as this links to the history too, and is just plain fun to do!

Your ds could use a general children's encyclopedia or the free online Wikipedia to research some more about the science topic, and then orally give you the things he learned and found most interesting - or if he enjoys being artistic, he could show you his research results in some artistic form with pictures, labels, etc.
As far as the reading goes, I know his dyslexia is a determining factor to what he does, but I just wanted to mention he could also do the Emerging Reader's Set, and start at whatever point you think is best for him. The books begin easier and get increasingly more difficult, so many moms have their older dc doing the set start in the middle of it or so. This works great!

Or, he could begin Drawn into the Heart of Reading Level 2/3 if he is a more independent reader already, but I'd ease into it.
For grammar, Beyond has a once a week lesson to introduce the basic parts of speech as well as other beginning grammar skills. If he has not had grammar, this would be wonderful! If he already has had an introduction to grammar, he could begin R & S English 2 (which is usually planned daily in Bigger Hearts). If he starts R & S English 2 this year with Beyond, you could just have him do it half-speed, spreading it out over this year and the next. Many moms (me included) have dc do a good portion of R & S orally, saving only about 1/3 or 1 section to be written.
I think both of your dc will love the history, along with all of its connecting portions of geography, poetry, Bible study, music, timeline, history activity, science, devotion, and art. Even if dc are doing another Scripture memory verse for church, it is very doable to sing the verse and do the short memory work each day. Our very active boys have especially loved this, and I've looked forward to that part of the plans with them as it not only hides the Lord's word in their hearts, but also "gets the wiggles out"!

Of course all of these are options, but I'd love for you to get the most out of your guide for the 2 of them, and each and every box of plans helps make for a very balanced day. HTH, and I'm excited for you to begin Beyond with your 2 sweeties! I think you'll find this board a very encouraging place to be too.
In Christ,
Julie