To leave an estate to your grandchildren and not your children you must have a written will. Along with a written will, it's helpful to have an additional "letter of instruction" to clarify the instructions in your will. Wills are legally binding and do not require a lawyer. But if you intend to bypass close relatives such as children when you pass on an estate, it is best to have a lawyer or another official party witness the will. If you die without a will, you will be considered "intestate" and the courts, not you, will decide who inherits your estate.
Research the laws of your state; state laws on wills and trusts can vary. In all states, an adult of sound mind can write a legal will. You must sign and date the will. In most states, the will must also be signed by two witnesses who have no financial interest in the will and who will not inherit any money. In your written will, make your wishes to pass on your estate to your grandchildren clear.
Write your will in compliance with the laws of your state and have it signed, dated and witnessed properly. If the will is not signed, dated and witnessed, or is not in compliance with the laws where you reside, it can be challenged in court by close relatives such as children who are not included in the will and who may have expected to inherit. Having an attorney witness the will is not required, but it can help prevent or defeat such challenges if they occur.
Be extremely detailed when drafting the will. Include all of your assets, real property, personal property, business assets and anything else that is part of your estate. If the asset is not specifically included as a part of the will, your children may claim it as your direct heirs. Be precise and consult an attorney if necessary.
Write a "letter of instruction" to accompany the will. In some states the letter of instruction is not legally binding but it can help clarify the intent of your will should there be challenges by your children after your death. In the letter, state very clearly that your intention is to leave the estate to the grandchildren alone and that the will should be followed exactly as written.
Name a property guardian in your will for grandchildren who are still minors. If you do not name a guardian for the assets you leave to your grandchildren, the parents of those children (your own children) will have control of the property until the grandchildren are 18.