Isn't it a joy and honor to share in your grandchildren's reading accomplishments? They are doing well!
The child who finishes McGuffey's Eclectic Primer is approximately ready to begin Level 3 of Little Stories for Little Folks. [It is important to complete all four levels of LSLF to be ready for CHC's second grade reading program.]
Some homeschoolers who begin with another reading program but switch to CHC still use LSLF for practice and enrichment reading, particularly for the family-friendly Catholic content and the sequential phonics development. [Library books can provide reading practice, but it may be difficult to locate books with any sort of phonics sequence, and almost impossible to find family-friendly, Catholic content.]
If you should decide that LSLF would be beneficial to your grandchildren, you may wish to begin with Level 2 for a quick review. [Since McGuffey's has a slightly different approach, there may be a few small gaps between the two programs. Moving back a level for a few days of review will help insure that nothing is missed.]
Someone has been doing a good job with those grandchildren!
God bless and guide you as you help form little souls for eternity,