The Children and Young People Bill sets out provisions to extend the age at which care leavers can get support from 21 to 26.
But the assessment process required to receive this "after-care" support should be scrapped and replaced with an automatic entitlement for every young person to remain or return to care up to the age of 26, children's charities say.
The coalition led by Who Cares? Scotland argues that this will provide significant benefits to young people, while saving money in areas such as housing, justice and health.
They have suggested amendments to the Bill which is being examined by Holyrood's Education Committee.
Duncan Dunlop, chief executive of Who Cares? Scotland, told the committee: "As parents, we look after a child through nappies to primary school to high school to college, maybe into further education. They leave home as a process, not a point in time. We are suggesting we should be able to afford this to our care leavers."
Those leaving care experience a "trapdoor shutting behind them", he said.
"We need to be able to continue to care for these young people up to the age of 26. What that means is to continue the care relationship. It is not necessarily suggesting continuing the care in a very high-cost residential bed to the age of 26."