The charity, which was founded by Esther Rantzen, launched an emergency appeal this week and warned that its overnight helpline will close unless money is received urgently.
"We are a lot smaller than people think, and £1m has a big impact," said media and PR manager Alison McKay. "We're only 19 years old and don't have huge reserves."
The charity's income rose from £12.6m to £12.7m last year, but donations have fallen by 30 per cent in recent months. "We can only assume it's due in part to the big disaster appeals such as the tsunami and all the activity surrounding the G8 summit," said McKay.
ChildLine made 17 staff redundant as part of a cost-cutting exercise last year. Another seven jobs on the night helpline, which received 8,920 calls between 9.30pm and 7.30am last year, are now at risk.
In 2003/04, the charity ensured that 1,859 children received help from emergency or social services.
Public donations account for 90 per cent of ChildLine's income.