The issue of pre-school is a complicated one.
Yes, as a parent of children who each 'lost' (or will lose) 2 terms of pre-school funding I do feel that that process is unfair. It means that the oldest children get the advantage of the most pre-school.
This is a natural consequence of the utter shambles that is both parties' policies on childcare funding. They can't make up their mind whether they are funding early years education (in which case it makes sense that it's term time only, but each child should be entitled to the same period) or childcare costs (in which case it makes sense that it starts from a fixed age point, but should be year round).
As an aside, it's also worth noting that, in areas where pressure on pre-school places is highest (e.g. my old area of London), children are often only accepted into the pre-schools for the academic year before they start school. So in that sense the playing field is level. Even in areas of the country where this isn't the case, many pre-schools have to prohibit children from joining part way through the year (because it requires holding the place open with no income for a big chunk of the year if the child doesn't start until Christmas or Easter), so again it is the affluent who manage to 'access' the right to that term or two terms, because they can self-fund from September until the funding kicks in.
Obviously the above refers to pre-schools rather than people who apply the 15 hours to their bill at a nursery. But there the younger children aren't losing the benefit of the time at the nursery in the same way.
Actually, as a December born, your DS would have been entitled to 5 terms of preschool, a delayed summerborn 6 terms, just one term more, whereas a summerborn entering at the expected age of just-turned-4 would have had 3 terms of preschool -
I'm not sure where you are going with this exactly. So your delayed summerborn gets 6 terms, but a January child gets 4. The maths always means someone loses out. The current system means your child gets two fewer terms than the 'best off'. Your solution means the a Spring child gets two fewer terms than the best off. You've just shifted the disadvantage around.