Who has Access to Irrevocable Special Needs Trust

We are faced with an account that is an irrevocable special needs trust account. There are two trustees. The beneficiary of the account is a disabled individual. The trustees are parents; the beneficiary is an (mid 30s) adult. When the parents pass away, who would have access to this account? Their lawyer couldn't seem to give them the answer. I came to the conclusion that the trustees were the only signers allowed access to the the account. The beneficiary could not access it, now or even at their death. Could you please shed some light on this?