I looks to me like this project is a perfect one for blocks that are not perfect! There doesn't appear to be anyplace where the seam need to match up. If one row is a centimeter taller than another, who is going to notice except someone with a ruler, and then they deserve to find the "error" The fabric on the ends will even out the width of the rows.
The important thing is the love and effort that the children put into this. You are just the facilitator to gather that love together!