About 40 firefighters tackled a blaze which gutted a children's play centre.
Fire broke out at the Funky Funhouse in Mercers Row, Cambridge, at about 22:40 BST on Friday, destroying the soft play area, cafe and day nursery.
No-one was inside at the time, but the company estimates damage caused by the blaze to cost about £750,000 to repair.
About 2,500 people were expected to use the centre over the long weekend, they added. The fire was started deliberately, an investigation found.
Aaron Othman, one of the company's directors, said it was "hugely distressing as everything inside is gone".
He said he believed the building had been broken into, and the safe and tills emptied, but was still waiting to see if the structure was safe to enter.
"It's the worst possible time," he said.
"We had eight birthday parties booked today [Saturday] and were expecting about 700 people.
"We're working hard to make arrangements with other venues so we don't disappoint the children."
The company had just completed work on a new day nursery within the complex, and was awaiting the "OK from Ofsted" before opening, Mr Othman added.
Fire crews were called in from neighbouring Suffolk to assist Cambridgeshire Fire and Rescue Service tackle the blaze at the centre, which is situated on an industrial estate close to a recycling depot.