This has previously been a knockout competition where children progress through the stages. This is common in sporting events where individuals or school teams play and the knock out process is well understood. In this way, it is not like entering the London Junior qualifying events (no qualification via school) and playing in more than one, nor like exam resits where you are only affecting your own result, not affecting another person's ability to pass.
How does a child, or parent, know that the child has failed to qualify because the event was a "strong" Megafinal, and not due to being a less experienced player? A child could have their hopes raised that they could qualify elsewhere, and end up being disillusioned because they enter several events and still don't qualify, rather than deciding that they want to come back and qualify next year.
Is there also a possibility that Megafinal organisers, worried about filling up with out-of-county players who have already played, will make theirs a "closed" event, disadvantaging children who genuinely cannot make their county Megafinal.
I see that our Sussex Megafinal organiser has just posted a comment voicing concerns of reaching capacity before local children have entered, and like her suggestion of a grade threshold.