Jack's Insects for children younger than 4th grade

I was just glancing ahead in the recommendation by Simply Charlotte Mason for science.  Next year, my son will be in 5th grade and Jack’s Insects is recommended.  However, it is only recommended for children in grades 4 through 6.  By this time I will also have a child in K5, 1st grade, 2nd grade and 3rd grade.  I have always done the same science for all the kids.  I realize this will eventually come to an end, but didn’t expect it this soon.  Would you do Jack’s Insects with the 5th grader and something different for the younger kids?  We just finished 106 Days of Creation this past school year and are beginning Learning About Birds this upcoming  year that we are about to start.  I have 6 kiddos and love the idea of keeping them together for science as long as possible, however, I am not committed to it if it isn’t beneficial.  I would love feedback from any moms who have done Jack’s Insects and also moms who have been faced with a child who is ready for more, but the rest of the kids are not.    What would you do or have you done and why?  Thank you in advance.

Regan