You need to treat your children fairly, and be able demonstrate them as adults that you did this.
However, fairly does not mean the same as equally.
So for each one, choose the school, out of those that are realistic financially and geographically, where you believe they will be happiest and will achieve the most, and be prepared to change this if this changes.
This may end up being the same school for all of your children, different for all or a combination in-between. You might narrow this by saying they will all go private or state, or you might not.
If your younger children end up needing a different type of education, (E.g. to pursue a talent or to support an as-yet unknown needs), you would not necessarily be giving them the best for them at the time they go to secondary if you insist on them having what is currently best for your eldest.
There are other threads running where a new headteacher has completely changed a school's ethos, so you could be doing your younger children a huge disservice by making decisions for them now as schools can change significantly in a relatively short period of time.