Some of the best websites to explain the sacrament of Eucharist (or Holy Communion)  – for parents, families and children. 

Together At One Altar is a resource for school-age children in years F-12 in Australia to develop their formation in the Eucharist and assist their full, active and conscious participation in its celebration – https://www.togetheratonealtar.catholic.edu.au/
A resource developed by Bishop Robert Barron, with the resources aimed more at adults – https://www.wordonfire.org/presence/