Many of the commercially available ‘Science Dictionaries’ use technical language that is not necessarily accessible for all our students. However, many textbooks have suitable glossaries that reflect the level of science being taught and could therefore be more appropriate. Griffith University has a free online science dictionary which seems to have reasonable definitions for junior students. https://www.dictionary.com/
In addition, the ACARA website provides a detailed glossary for the Science Curriculum, which you can find here https://www.australiancurriculum.edu.au/f-10-curriculum/science/glossary. This could be printed for students or used as an online resource.