I want to be clear here: It does not matter what you say in your will or trust; the beneficiary document attached to your IRA accounts and your life insurance policy overrides what you say elsewhere. If you want to change the beneficiary, you must change the beneficiary document.
I'm a big advocate of a work-for-pay setup rather than an allowance that isn't attached to chores - it's a great way to impart the value of money to your children.