"The top marginal personal-tax rate is now above 50 per cent in most provinces, while the small-business-tax rate is about a third of that."

This is an incomplete comparison.  Small business corporate tax is about a third of top personal marginal rates, but small business dividend tax needs to be paid on top of the corporate tax.   Those dividend rates have been established to roughly make the dividend plus corporate rate = personal income tax rates.  Yes, dividends can be paid to lower income family members, such that the combined tax burden on that portion is lower.  

If 'income sprinkling' is the primary concern, a more incremental and less controversial proposal may have been to redistribute the dividend tax rates (increase rates in the lower brackets, decrease in the mid and upper).  This could reduce the incentive or tax breaks associated with family dividend payments, without increasing overall small business tax burden.  But as it stands, this proposal is a hasty tax grab.