I am slightly confused by the way you have worded your post. Are you saying that one of the 5 beneficiaries is your disabled child and you are looking to establish a DOV and Trust so the money does not go directly to him / her; or are you saying that you are the beneficiary and you are looking to divert the money to a Trust Fund for your child rather than receive it yourself?
If you are the beneficiary, and are in receipt of benefits, then it is quite possible that the Benefits Office will regard diverting the money to a Trust as deliberate deprivation of capital, but I would check this for certain with someone like Citizens Advice Bureau.
If you are not in receipt of benefits yourself then I am not sure why you feel you need to go to the expense of a DOV and Trust now. It would be perfectly possible, and indeed best advice, to write your will (and that of any spouse or partner) so that any legacy to your disabled child is paid into a Disabled Persons Trust with your child as the beneficiary. Under what is known as CRAG guidelines the capital within such a Trust is not taken into account for benefit purposes, although there are rules regarding what the Trust can and cannot pay for, e.g. it can pay for holidays but not for day-to-day living expenses that any benefits being paid are designed to cover.