Try some of these resources for when you need a brain break! The majority have free versions or free trials. You do not need to sign up for anything, these are just some ideas.
Headspace: https://www.headspace.com/meditation/kids
Go Noodle: https://www.gonoodle.com/
STand Up Kids (Movement Breaks): https://standupkids.org/movement-break/
Insight Timer: https://insighttimer.com/meditation-topics