JKR's net worth is less than a billion, which means she's probably not raking in over a billion annually.
Personally, I'm always dubious of the rich and famous genuinely finding unmet cases for charitable organisations. Especially when they've made a fortune outsourcing being morally dubious to others - she can save children because others are paying her to be allowed to sell low quality merchandise almost certainly made in exploitative conditions.
She's not alone, there's many more e.g. Messi donating lots to children's cause through his own charitable organisation after gladly being a global ambassador for unhealthy snacks targeted at children.
That's a long way from advertising unhealthy snacks.
Lego isn't made in dubious conditions, so for toys she's already above average.