It seems as though that person isn't a single mom who's relying completely on their child and spousal support to make ends meet. If that goes to zero, that bit of extra money they're getting from the child tax benefit is not going to be enough. Maybe they could live in their car and continue to feed their children, but how can they feed their children and provide a house on the child tax benefit that's being provided to them?
The child tax benefit is assuming that there's some other base form of income for that family, whether it be the CERB or the wage subsidy. It's not assuming that the family income has gone to zero.