is there a legal form letter of "Acceptance of New Trustee" for a Trust for a child of special needs?

There is no specific form.  If the trust is done as part of an estate, you can get a certificate of trusteeship from the surrogate's office.  Otherwise, just have the Trustee write something indicating that they accept the role.  I usually have the Trustee sign the SNT along with the grantor of the trust, but if that was not done, a simple letter will suffice.