They contribute large annual bonuses to a donor advised fund. This both reduces their own taxable income and pays tithing at the same time. Parents get to the point that the donor advised fund has more money in it than they will ever owe in tithing. You can transfer funds between donor advised funds belonging to two different people without tax consequences. So the funds get transferred to children who use it to pay their tithing. Children don't get a tax deduction for the donation but it does occur in their name. Since the children are younger and likely in a lower tax bracket this reduces the larger family's aggregate taxes.

I've also seen this strategy used so children still "pay tithing" while saving their own tithing money to start paying tithing every other year. To take advantage of the itemize every other year trick.