Dr Hanna, who is interim head of the trust's Evelina London Children's Hospital, said up to half of all planned treatments could be cancelled as the trust redeployed its senior doctors - and this is on top of the impact of postponements during previous walkouts.
She said the cumulative impact of this was particularly problematic for children who are growing and developing.
NHS Providers, which represents health managers, said such concerns were widespread.
Chief executive Sir Julian Hartley said: "It's clear from our extensive dialogue with trust leaders that we are in uncharted territory.
"We need a solution to prevent further strikes and we need it now."
The organisation also released a series of statements provided by hospital chief executives.