The typical size of such loans reached £1,545 in August, Aviva's family finances report found, compared with £701 in May, the highest figure the study has recorded since the reports began in January last year.
The report suggested the rise was an indication that people are turning to their loved ones to avoid taking on more formal borrowing and the charges that come with it.
It also found that family debt levels crept up from £9,314 on average in May to £10,563, excluding mortgages but including debts such as personal loans, overdrafts and credit cards.
Meanwhile, average family incomes fell from £2,150 in May to £2,003 by August, suggesting some parents are taking unpaid leave or cutting down on hours to care for children during school holidays, the report said.
The figures also showed a drop in the proportion of more "well-off" families. The share of families with an income of more than £2,500 a month fell from 36% to 31% year-on-year.
The typical family's savings fell to £1,131, from £1,228 in May, but they remain higher than the same period last year when average savings stood at £982.
Louise Colley, head of protection sales and marketing for Aviva, said: "As every parent knows, school holidays can be a financial challenge, particularly for workers who don't receive paid leave.
"However, it seems that families are trying to cut their cloth accordingly and the fact that some have dipped into savings this quarter suggests they may have been planning ahead."