For those grades, I would plan lots of nature study outdoors and field trips like going to the zoo, aquarium, pet shop, etc. Go to the library and pick up animal books they are interested in. In grade 2, my son read lots of non-fiction animal science books in his free time. I had him reading the Christian Liberty Press Nature Reader for school science. When he was in 1st, we tried Apologia. But it was too much.
I have decided now that living books and nature study alone are all that are needed until 4th or 5th grade, when they can pick up an Apologia or other elementary science text in preparation for a jr. high science text. Or they can keep on with just living books and pick up an elementary science text for 7th and then a jr. high text for 8th before going on to high school science. My son will be doing The Sciences by Holden for grade 6, along with related books like Eric Sloane’s Weather Book. He can pick an Apologia elementary text for grade 7 and then do Apologia General Science for 8th.
Please read what Karen Smith of SCM has to say on these posts:
https://simplycharlottemason.com/scmforum/topic/living-books-038-nature-study-only/
https://simplycharlottemason.com/scmforum/topic/science-advice-please/
There are also many ideas for living books to use in the above links. We did Outdoor Secrets one year and really liked it and the companion books for it.