More than 100 children a week are contacting the ChildLine helpline with worries about their parents' drinking or drug use, according to the NSPCC.
It said in the year to March, 5,700 children had called, but founder Esther Rantzen told the BBC there were likely to be many more too afraid to do so.
Two-thirds of those callers had mentioned their parents' drinking.
The children calling about that issue were also more likely than other child callers to report abuse, it added.
Ms Rantzen said: "These are the children that know our number and ring us, but what about the many, many thousands of children who aren't, alas, familiar with ChildLine's work and who might be fearful of ringing us?"
She added: "I am, in a sense, imploring those people who work with children to be alert to the possibility that the silent, friendless child... may have trouble at home created by alcohol and drug problems."
An NSPCC report detailing the figures suggests the children, some of whom were as young as five years old, were often trying to cope with the role reversal of attempting to look after their parents, brothers and sisters.
The helpline, which is run by the NSPCC, received more than 150,000 calls during the year.
Although concerns about parental drug and alcohol abuse made up only a small percentage of them, the head of ChildLine, Sue Minto, said: "The fall-out from parental drug and alcohol abuse is a ticking timebomb in many children's lives.
"It's vital these children are helped before lasting damage occurs."
She said children living with parental alcohol and drug problems were at more risk of harm than other children and ways needed to be found of helping them sooner.
"But we must also remember they can be fiercely protective of their parents," she added.
"One young girl's first comment to the ChildLine counsellor was: 'I don't want anything to happen to my mother'."
The NSPCC also cited the example of one 10-year-old, who said: "My mum drinks all the time. She leaves me alone lots of the time. I feel scared and lonely.
"I look after mum when she drinks and put her to bed. She shouts and hits me. I don't want to feel pain. I want to die."
It said more than 4,000 children had rung the helpline during the 12-month period to say they were worried about their parents' excessive drinking.
Thirty-five per cent of those had reported suffering physical abuse, which was more than three times the rate among other children who called.
'Episodes of violence'
Twenty per cent mentioned issues with family conflicts, while 10% spoke of sexual abuse.
Ms Minto said: "Some children told ChildLine about their parents' severe mood swings, episodes of violence and emotional instability.
"Some said their parents were regularly sick and that caring for them had affected their schooling, or prevented them forming friendships with other children."
Chris Sorek, chief executive of charity Drinkaware, said: "Lots of parents might be horrified to learn of the number of young people seeking help as a result of parental alcoholism, but the news should serve as a timely reminder that you don't have to be an alcoholic to have a direct impact on your children.
"Regularly drinking to excess in front of children will only normalise alcohol misuse but it's important this pattern within the family unit is broken to ensure young people grow up to have a healthy relationship with alcohol."