Callum Reid, of Lacy Street, Hemsworth, and another youngster from St Helen's CE School, on Moorshutt Road, made their getaway at lunchtime last Wednesday.

Callum's parents, Ryan and Chloe Reid, said the school noticed them missing when register was taken after lunch and the boys were later found by a member of staff near R&S Waterson coach firm, on Highfield Road.

Mrs Reid, 21, said she only found out when she came to collect her son at the end of the day.

She said: "I was shaking and crying. I was in hysterics. At first I was angry at Callum for going out – but then I thought, what could they have done to prevent it?

"It's out of order. He crossed a main road unsupervised. Anyone could have picked them up."

Mr and Mrs Reid called for security to be tightened following the incident.

Mr Reid, 23, said: "They walked all the way around the building and through the main reception gates. The school said they couldn't say how long they were missing for."

Mr Reid said the boys should have been watched more closely after they attempted to escape the day before.

He added: "They got stopped by dinner ladies and admitted they were trying to get out. Callum thinks it's a game. Any four-year-old enjoys a bit of excitement. I'm not very happy. He could have been run over ... anything could have happened. It's quite a busy road outside, with a bad corner."

Sue Henderson, headteacher at St Helen's, said the school had followed correct procedures concerning lost children – but said additional security measures had since been taken.

These include improving communication with families and ensuring staff double-check entrance gates are closed at breaks and lunchtimes.

Reception children will be kept in the nursery outdoor area, which is fenced in, and an extra staff member has been added to the team in reception.

Mrs Henderson said: "The safety and welfare of the children is always our main priority and although situations like this do not happen often we are well prepared and have protocols in place, agreed with the education authority and governors.

"We have a lost child policy to deal with this type of situation and this was fully followed by all staff last week. However, we are not complacent and have looked to see how procedures can be improved. As a result we have additional measures, which will help make security more robust."