If your witness passes away, your will remains valid even if the witnesses have passed away or become invalid to stand as witnesses. Their testimony stands that at point of signing, the testator was of sound mind. Usually before grant of probate, court may ask witnesses to validate the will but if they are unable to do so, will is valid but could be prone to questioning. For practical reasons, the will should be signed again with a fresh pair of witnesses. If his beneficiary also passed away, the executor will distribute your share of assets across the remaining surviving beneficiaries according to their allocation. For example, 34%, 33% and 33% to each of 3 kids. If child 1 passes away, the 34% will be distributed to the remaining two children following their new allocation percentage, which would be 50% and 50%.