Background: Social media is an important communication tool during times of crisis because of its vast reach. Understanding the effectiveness of sharing public health guidance and promoting schoolchildren's physical activity during the COVID-19 pandemic can inform dissemination best practices. Methods: We classified 418 posts from parent/community members of a school-based physical activity Facebook group by content type, and used concurrent mixed methods to examine (1) differences in dissemination effectiveness (reactions, shares, and comments) between two pandemic phases and (2) themes and sentiments of comments. Phase I included school closures through the release of national school re-entry guidelines (March 1, 2020 - May 15, 2020) and Phase II extended through the school year start (May 16, 2020 - August 1, 2020). Results: Policy and guidance posts prompted more comments while feel-good stories produced more reactions compared to other content types. Members reacted more during Phase II, which mainly consisted of policy and guidance (86%). Four major themes of information and resources, personal disclosures, questions and concerns, and support for educators emerged. Conclusions: Sharing public health guidance for schoolchildren on social media was an effective strategy to disseminate information when in-person discourse was limited. Creating social media spaces where discussion is encouraged can provide social and emotional support for parents/community members.