Girlguiding UK is concerned that Government plans to improve road safety will have a major effect on children attending after-school activities such as youth clubs, Brownies or Cubs.
The consultation on amending the compulsory seat belt and child restraint requirements in cars proposes that children under a certain height - likely to be set at either 4ft 5in or 4ft 11in - will need to use a special restraint or booster seat.
The charity has warned that if the taller option is chosen, 407,000 girls who are members of Rainbows, Brownies and Guides - 75 per cent of its young members - will be affected.
Jennie Lamb, marketing and communications manager at Girlguiding UK, said that not enough parents had been made aware of the issue, or of the fact that booster cushions cost £10 each.
She said that where parents collect others' children for after-school activities, multiple booster seats may be needed. "This may be good for a long journey, but the school run or the trip to Brownies is not in the same league," she said. "Most of those people don't get above 20 miles per hour."
Last month's consultation was sent to organisations including Barnardo's, the Scout Association, the Boys' Brigade and the Health and Safety Executive.
A Department for Transport spokesman said: "We want to save children's lives, so it is a balancing act."