Whilst there are several options for these beneficiaries depending on your circumstances, one of the most common is to include a Protective Trust in your Will.
A Protective Trust is basically having someone else look after your adult child's money on their behalf- this person is called the Trustee. The Trustee receives the money from the estate and would usually invest it conservatively. They cannot use the money for themselves, but must use it for your adult child.
You can customise a Protective Trust according to your wishes. In a full Protective Trust, the Trustee has full discretion about how to use the money to benefit your adult child. However, you could customise this to say, for example, that your adult child receives all of the income from the invested money with no questions asked, and the Trustee has discretion about how to use the capital. In another example, the Trustee may have discretion to use only 10% of the capital in any given year.
There are many factors which go into deciding how to structure a Protective Trust including the age of your adult child, the nature of the reason why you want to set up one in the first place, and who is available to act as a Trustee. We can discuss all of these with you.
The key point for you to remember is that there are many options to make sure a vulnerable beneficiary is well looked after in your Will, and we can assist you with this.