Tustin was jailed for life at Coventry Crown Court on Friday, with a minimum term of 29 years, after being found guilty of murder.
Hughes was sentenced to 21 years for manslaughter.
The Attorney General’s Office has confirmed the sentences will be reviewed and Education Secretary Nadhim Zahawi is due to make a Commons statement on the case later today.
Arthur’s maternal grandfather Peter Halcrow, 61, from Dunkeld, Perthshire, said Tustin and Hughes should never be allowed out of prison.
He told The Sun: “They must never see the light of day again. No punishment could ever be enough for this pair.
“I have never favoured the death penalty because I know mistakes can be made by courts, but in my view they have forfeited their right to live.
“It will burden taxpayers but, as we don’t have capital punishment, they should certainly never leave prison as long as they live for such cruelty and inhumanity.”
Arthur’s grandmother Madeleine Halcrow was among those who gathered outside his house in Solihull to remember him.
She wore a t-shirt with a picture of his face and wiped away tears as the crowd released balloons into the sky.
An independent national review will identify the lessons to be learned from Arthur’s death, in June last year, for the benefit of other children elsewhere in England.
The government has also commissioned an urgent inspection of social care, health, police and probation services in Solihull to whom Arthur was known, in what is effectively an upgrade of an existing local review.
It emerged in court the boy had been seen by social workers just two months before his death, but they concluded there were “no safeguarding concerns”.