Whether you like it or not, sooner or later full time study mode will be established in most primary schools. The students that have to change from half day to full day midway during their primary school life will face difficulties and school teachers' and parents' assistance are extremely important.
On other hand, I think parents that have kids studying or going to study in half day primary schools should guide/supervise them to use their extra time (after/before school) effectively
The only thing I prefer half day study to full day study is parents can spend more time with the kids (especially for parents that no need to go to work or parents that are working on shift duties)