London mayor Sadiq Khan has called on the government to allow under 18s to continue to get free travel after demands it be withdrawn were made part of £1.6bn lockdown bailout conditions.
Children are currently entitled to Oyster cards allowing free or cut price travel, depending on their age.
Mr Khan said stopping it under social-distancing guidelines on overcrowding would "hit the poorest hardest".
The government said it was working with transport authorities to reduce demand.
Transport for London (TfL) previously announced that free travel for children would temporarily end under conditions of the government's continuing £1.6bn deal.
In exchange for the emergency funding, transport bosses also agreed to start collecting fares again on buses, restore a full Tube service as soon as possible, and temporarily suspend free travel for over-60s during morning peak hours.
The congestion charge was also brought back earlier this month and it will be subject to a rise in cost, from 22 June.
In a letter to Transport Secretary Grant Shapps, Mr Khan said he agreed with the "need to reduce the numbers of children using bus services in particular", but felt suspending free travel was not "the right thing to do".
"It is abundantly clear that losing free travel would hit the poorest Londoners hardest at a time when finances are stretched more than ever," he wrote.
TfL has also calculated that about 30% of children who currently travel by bus to school would still be "eligible statutorily for free travel", according to Mr Khan.
The mayor said the "significant" cost of this would fall on local boroughs while also undermining "the effectiveness of introducing bus fares for children as a way of reducing bus usage".
London's deputy mayor for transport, Heidi Alexander, is due to meet government officials next week about the matter.
Mr Khan added he hoped the discussions "can focus on the overriding objective of minimising public transport crowding at peak times and particularly the role that more walking and cycling to school can play in that".
A spokesperson for the Department for Transport said the government was working constructively with TfL on ways to reduce demand on the network... including looking into the option of temporarily suspending free travel for under 18's.
"We have also published clear advice that urges people to avoid public transport if possible, and announced £2bn in funding to encourage even more people to begin cycling and walking."