please explain how Im wrong?
Seriously? Can you not look beyond the end of your nose and at the bigger picture?
In a persons lifetime they will generate a certain amount of tax from income,vat, duty etc, they will also potentially incur costs from healthcare, education and social services (let's gloss over general rerunning of the country/defence/infrastructure etc) the government averages out the cost and balances the books (let's also gloss over the fact that there is hardly ever a surplus and there is a huge deficit/money printing)
Having children or not has nothing to do with it. You could be a single entrepreneur generating several million in revenue for the government and die a quick natural death or be a workshy fat cretin spawning several equally workshy cretins and costing the U.K a sum in lost revenue (from theft,black market etc) and state benefits paying zero income tax and just a bit of alcohol and tobacco duty.
Your kids may end up taking from the state and the single person paying for their ill health, it's not as simple as you like to think.