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It will be held at St Leonards School, close to their home where they were killed by father Edward Livingstone on January 15.
"We'll remember their quirks and all the happy times and funny memories, and we'll sing their favourite songs," a statement from the school said.
"The dress code is bright colour only - Ellen would approve."
Hundreds of people are expected at the 2pm service and a marquee has been put up to accommodate everyone who wants to pay their respects.
"Last week an immeasurable sadness settled over our community and at times it threatens to overwhelm us," the statement said. "But tomorrow we're going to do our best to set that feeling aside for a wee while and we're going to be kind to ourselves.
"We're coming together as a community to remember the joy that Bradley and Ellen brought, to show [their mother] Katharine that she is not alone and is in all our hearts, and to help our children start to come to terms with what has happened to their friends."
Bradley, 9, and Ellen, 6, were shot dead in their beds by their father who then turned the gun on himself.
Donations to a bank account for Ms Webb had topped $12,800 yesterday.
A TradeMe auction organised by school alumni will go live next Wednesday.
Organiser Tim Foley said the response from the public had been "amazing" with dozens of items donated.
He was confident "thousands of dollars" would be raised. Proceeds would go to the school.
"The school and community of St Leonards want to thank everyone in Dunedin and farther afield that have been so unhesitatingly generous," the statement said. "If it hadn't been for the support of those individuals and organisations, we wouldn't have been able to cope. Your kindness has been incredibly moving.
"We'd also like to thank the press who have at all times been courteous and respectful and have been a critical link in helping to get the family and the school the support they needed."
Donations for Ms Webb can be made into ANZ account 06 0909 0401 308 00.