If you're talking about a large sum of money, see an attorney or CPA before doing anything. If you "write a letter" explaining this wish and that is considered a disclaimer, you might have what is known as an "indirect" skip and the Generation Skipping Tax may be implicated.
When you are a Bear of Very Little Brain, and you Think of Things, you find sometimes that a Thing which seemed very Thingish inside you is quite different when it gets out into the open and has other people looking at it.
--W. T. Pooh (aka A. A. Milne)