Well, it helps the other kids because it creates a more level playing field, when it comes to competition for university places and access to desirable professions where private school educated kids are disproportionately represented.
When access to private education and the advantages it confers (connections, superior education and facilities, a distraction free education, confidence, access to better universities and jobs) are determined by wealth of parents rather than ability or potential of child, that is unfair on all children, and entrenches privilege within society which is ultimately bad for all of us. You get a overclass of privately educated judges, lawyers, politicians, journalists, CEO’s, bankers etc. wielding power and influence over the rest.
It also helps all kids because if fewer people send their kids to private schools, those parents who would have gone private and who are proportionately wealthy, and prioritise education and (presumably) intend to raise kids who will also prioritise education, will be in the state sector, enriching it by their presence, raising standards, inspiring their peers, campaigning for flute lessons and raising money for the PTA.
I’m sorry that your individual child’s education was disrupted by this change, but at a societal level I think private education entrenches inequality and should be discouraged.
Political policies that are intended to improve the greater good, usually do disrupt life for individuals, while hopefully making things better or fairer overall. I’m personally going to be disadvantaged by the inheritance tax changes, because my PIL’s who built up a business from 2 - 30 people will be stung by it. But I still think fairer distribution of wealth is a good thing overall, even if I personally will be poorer for it.
I could send my kids to private school, PIL’s would fund it in a heartbeat, but I don’t, because I don’t want to buy a demonstrably unfair advantage for them, and I think a little reality while they are growing up, is probably a good thing.