One of the problems with having a morning children/afternoon children split in primary is the fact that young children's attention spans vary at different points in the day.
Maths and English (arguably the 2 core subjects which we would want as a minimum for our kids) are usually taught in the morning lessons because that is when young children are most attentive.
I know from experience that getting children to concentrate past about 11am is a losing battle. After lunch is usually reserved for IT, PE, science, art, assembly, ect.
We'd have to be alternating between morning and afternoon sessions for each child to be fair.