All the posters saying you can't spend thousands on some children and not the others, and it's so unfair and the younger child will be resentful etc are entirely missing the point.
This isn't about the OP deciding her younger child isn't worth the same as her others, or not wanting to spend the money.
This is about a hugely changing financial climate, the reality that private school fees are now 20% more expensive than they were just one year ago, and what was feasible for the two older children at the time they were in Year 7 now simply isn't feasible for the younger child without needing to sell the family home. (Some people need to read the OPs posts - she has made it clear this will be the outcome if they pay for the younger child to go to the same school).
No sensible parent would surely consider it appropriate for their children's wellbeing to sell their home, reduce the quality of life of all three of their children, and potentially put themselves into debt just for the sake of 'fairness' to the younger child.
It doesn't make sense and the younger child can have this explained to them in a child-friendly way. They will benefit far more from being able to stay in their home, have financially unstretched parents, and the opportunity to do plenty of extracurriculars and enjoy holidays and treats.
A private education is an unnecessary luxury. The younger child is not being denied an education. The drama on this thread is ridiculous.
They can't really afford three sets of private school fees, That's the reality. Not without substantial changes to everyone in the family's way of life.
If you can't afford it, you can't afford it. There is no real choice in the matter here. Having to sell your house means you can't really afford it, let's face it.
And for those saying pull the other two out now - that's not feasible either. The older one is half way through their GCSEs. You can't just switch schools at that point because there's no guarantee the local state is doing the same exam boards or syllabi, and the child would be at a huge educational disadvantage.
You could take the child in Y9 out now to enter into Y10 at the local state, but it's not guaranteed there would be places - and the problem with that is also everyone would have made their GCSE choices already and there may not be the same options available for this child.
The younger child faces no disadvantage in going to a good quality local state with their friends. They would not be disrupted in any way by this.
The older children would be hugely disadvantaged by being pulled out of a school they are already happy in and are about to finish or start exam courses.
The parents need to make the right choice for ALL their children - and that doesn't mean that all the children will have the same options. That's just the way life is sometimes.