This is my go to for developmental milestones for school agers. Maybe you can see if someone you know has it? or a library? or maybe your school could purchase one for all the teachers to share as it covers birth to age 12. https://www.amazon.ca/Developmental-Profiles-Pre-Birth-Through-Adolescence/dp/130508831X
For the milestones... How about a textbook from my human development degree?
Developmental Profiles: Pre-Birth Through Adolescence https://smile.amazon.com/dp/130508831X/ref=cm_sw_r_cp_api_glt_fabc_PW6S50B1TVHWAB046YB4