choosing a delightful reading program

HI!  I’m new to Simply Charlotte Mason.  This will be the second year of homeschooling for my family.  I have a 7 year old boy who is a rather reluctant reader.  He knows letters and sounds, but is having trouble putting them together into words.  I just received the Charlotte Mason Teacher manuals and It suggests Delightful Reading Level 3.  I’m really not sure which to choose because it seems as though Level 2 may be more appropriate.  I also have 4 year old twin boys and I believe they could also benefit from Level 2…at the same time I do want to provide a challenge to my 7 year old.  Please advise!  Thank you!