Melissa,
I'm also thinking LHFHG for your younger child, probably with A Reason for Writing K and one of the phonics programs. You could use either The Reading Lesson or Reading Made Easy. As far as math goes, if she's already had a formal year of K math, then you could use 1A/1B for math. If she hasn't really had a formal K year of math, then you could do Earlybird Kindergarten Math instead.
For your will be 8 and 9 year old, I would probably lean toward Bigger Hearts. Your will be 8 year old should be able to use Rod and staff English 2 as scheduled in Bigger and use either List 2 or the first level of studied dictation (both of which are in Bigger Hearts' guide).

You will want to be sure that your will be 8 year old has completed phonics, and then you can head into the Emerging Reader's Set, which is scheduled daily in the Appendix. Bigger includes hands-on lessons for Singapore 2A/2B, so you will be set for your will be 8 year old's math.
As far as your will be 9 year old goes, you will want to use DITHR for reading. I would lean toward Level 2/3 Student Book for her in DITHR, unless she has had quite a bit of formal literature study already. You could choose the Level 3 or Level 4/5 Book Pack for her to use with the 2/3 Student Book.

There is a schedule in the Appendix of Bigger Hearts for Singapore math 3A/3B, so this should work well for your will be 9 year old.
I would use English 3 with the 9 year old after coming out of PLL. This is a level ahead of what is scheduled within Bigger but will fit your child better.

For spelling, depending on how well she spells, you could begin at the beginning or mid-way through the studied dictation passages provided in Bigger Hearts. Since she is new to studied dictation, but is a bit older, I'd likely begin in the middle of the passages and see how she does. The directions for studied dictation are at the beginnning of the passages in Bigger's Appendix.

The extensions in Bigger are technically for ages 10-11, so you can weigh whether you want to add those to your will be 9 year old's day or not.
Both the will be 8 and 9 year olds could do cursive as scheduled in Bigger if you feel they are ready.
If you have more questions, ask away!
Blessings,
Carrie