Some ideas:
~Fit them in with the topic you are reading
~Let them be free reading picks when you are reading to the younger ones
~Schedule your read-alouds (like a 15 minute daily picture book reading type thing) to include a day or two a week with these other science topics (one of these types of books only takes 3-5 minutes to read-aloud generally)
~They can be free reads in a reading basket