The NSPCC said it received more than 1,200 calls and emails last summer about children being left unattended - up around a third on the previous year.
By law, there is no minimum age at which a child can be left alone, but parents can be taken to court for neglect if a child is at risk of suffering or injury.
Childcare pressures can make the summer holidays a stressful time for parents and carers, the NSPCC said, and deciding whether a child can be left is a difficult decision.
Between July and September last year, the charity's 24-hour helpline received 1,294 calls and emails from UK adults asking questions or raising concerns about youngsters being left alone.
This is up from 949 contacts during the same period in 2015, the charity said.
Some of the calls and emails received last year were from people asking for guidance on when a child can be left unattended, but 83% were considered so serious by the NSPCC they were passed on to the police or social services.
In some cases, people reported youngsters being left alone overnight, or young children being left to feed themselves, the charity said.
NSPCC chief executive Peter Wanless said: "Deciding if a child is ready to be left on their own can be a very difficult decision and the summer holidays can be a difficult time for parents and carers as they face increasing childcare pressures.
"Although the law doesn't specify a minimum age, no child should be left on their own if there is any risk they will come to harm.
"Children mature at their own rate so it's really important parents think carefully about what is right for their child.
"Children shouldn't be left on their own if they are not happy with being left, or if they don't know what to do in an emergency."
The NSPCC said it is calling on parents to read its home alone guide, which includes questions mums and dads should ask themselves, and their children, before making a decision about whether a youngster can be left alone.