To soften the blow for the rich, it'll be introduced at the same time as making another large increase. There will undoubtedly be loud protests on two matters of fundamental principle. First, some will claim it breaches the Beveridgean principle of universalism. The Chancellor will reply that as long as it's still paid out to all mothers, rich and poor alike, it will still be a universal benefit, even when it's taxed back at the end of each year. That's important, since even in well-off families fathers can vanish, leaving mothers penniless for weeks.
Child benefit acts as a universal safety net for all children in a crisis.
The other principle it raises is women's independent taxation. Feminists say that if either partner can be taxed on it, that's a step back to joint taxation. But it doesn't mean returning to joint tax forms. Those who worry about women's financial rights should be lobbying the Chancellor on the real injustice of the way poor women are treated in the social security system. Any woman living with an unemployed man loses all her own benefit rights.