But now immigration officials may expel Kerli, a 24-year-old Estonian au pair, even though health experts, doctors and Sebastian's head teacher say her presence is vital for his continuing progress.
Some of Sebastian's previous carers stayed for weeks; some stayed only for a few days. Kerli has been with him two years so far. It is the length of her stay, say Sebastian's parents, that has helped him so much. But two years in Britain is the maximum permitted for an au pair, so Kerli has attracted the attention of the Home Office's Immigration Department.
Despite written pleas from medical experts, Home Secretary Michael Howard has ruled that Kerli's case does not justify "exceptional treatment" under immigration rules and that she must leave Britain immediately.
Sebastian's father Trevor Syrad, who runs a human resources management consultancy, has lodged an appeal. Kerli can stay until it is heard.
"I find it very difficult to understand how they can judge Kerli's case not to be exceptional circumstances. I don't feel the authorities have used their discretion, they've just made an arbitrary decision," he said.
Mr Syrad, 41, from Oxshott, Surrey, and his wife Kristina, 38, who have three other children, have fought hard to provide the best care for Sebastian, who was born with a a cleft lip and palate, since he was diagnosed as mentally retarded at the age of eight months.
The couple embarked on an intensive programme to stimulate Sebastian physically and mentally. This involved two full-time carers plus 30 local volunteers who came in regularly to help give the baby up to nine hours' attention a day.
When Sebastian was three, the treatment was changed to a one-on-one approach to improve his communication skills - and that is when the Syrads had trouble finding someone to stay for longer than a year.
"We had young women staying with us from various countries and also from Britain. Some stayed for a year, others left much more quickly," said Mr Syrad. "Soon after Kerli arrived she won the trust of Sebastian. For instance he does not pinch or hit her at all, as he has done with some of the other girls. When we have had other girls as well as Kerli, Sebastian might go to them and say 'I don't like you, I like Kerli'.
"She takes him on walks in the woods and is very good at talking to him naturally. She has taken him to Estonia twice, and he seemed to have a good time."
Mr Syrad said the progress made by Sebastian since he had gone to the local primary school and been with Kerli had been enormous. "His mobility and communication ability have improved dramatically over the past 18 months. A large part of that is down to Kerli and the quality and continuity of attention she has provided."
If Sebastian were to deteriorate and had to go into a home or even a special school, this would cost the state "thousands of pounds" said Mr Syrad. "As it is Kerli costs the state nothing. The irony is that we are as a family trying to uphold decent family values and trying to keep the family together. But this is the response we get. It is very frustrating."
Kerli, a former student, said there was no question of her trying to get round rules just to stay in Britain longer: "I don't really see my future in Britain but I want to help Sebastian and here is a chance to do that."
The Syrads have the support of Sebastian's head teacher, Angela Ewing of St Andrew's primary school, Cobham. "He deserves all the support he can get and continuity of care is very important," she said. "He is a lovely child."
Last Wednesday the family and Kerli lodged an appeal backed by letters of support from the boy's consultant paediatrician Dr Barry Lewis, from Surrey County Council's educational psychology service, from their GP and Mrs Ewing.
Claude Moraes, director of the Joint Council for the Welfare of Immigrants, said the whole immigration policy surrounding au pairs was archaic, and sexually and racially discriminatory.
"The Home Office does have some discretion and if this woman is helping the health of a child and is willing to extend her stay then that should happen. But the Home Office wants to look tough with the new East European states on immigration policy.
"This case brings it home to a wider community just how discriminatory immigration rules are and how inflexibly they are applied."
A Home Office spokeswoman said the Secretary of State had carefully considered the case but decided that there were no "exceptionally compelling circumstances" which would allow Kerli to stay.
She said all au pairs came in on the strict understanding that their main purpose was to learn English, not to work or provide care, and that they all undertook to leave after a maximum of two years.Reuse content