The bodies of eight-year-old Hannah Marie Flitcroft and five-year-old Michael were found with their mother, Mary Ann Flitcroft, 35, by a passer- by who spotted their car in Harpsden Wood near Henley-on-Thames early yesterday.
Paramedics and police officers tried to revive the family but they were all dead on arrival at Royal Berkshire Hospital.
Thames Valley police said they were treating the deaths of the children as suspicious. It is believed they may have been killed by car fumes.
Neighbours of the family, from Woodley, in Reading, said they were "shattered". They said Mrs Flitcroft had been separated from her husband for nearly a year and had suffered depression. She had also been struggling to cope with the recent death of her mother.
The two children had been due to go back to Rivermead Primary School, in Woodley.
A neighbour, Bill Brooks, 67, said: "The first we knew about it was when two police cars pulled up outside her house. She must have driven her kids off in the middle of the night.
"It is such a tragedy. We were aware she suffered some sort of depression and often said she didn't feel well, but we had no idea that things had become so bad.
"I don't think the separation had much to do with it because they had been on friendly terms and he often came to visit the children.''
His wife, Jane, 65, said: "I think she just felt alone and there was no one to pick her up. They were very nicely spoken and behaved beautifully. It is just so awful that the kids were involved."Reuse content