It is always best to tell them as soon as you are able to, as they will only be able to support your needs as an employee with things like flexible working hours and time off if they are aware that you are the parent of a disabled child.
There are also other people you may want to tell, such as neighbours, other parents at school if your child attends a mainstream school, and colleagues at work. Unfortunately, some people will have a negative reaction when, or after, you tell them, but these people are in the minority. If you do experience this, remember there are people who can support you, such as the Mencap Helpline, the police, your local citizen's advice bureau, workers unions and social services.
Remember that in most cases those you tell will be supportive of you and your family and any worry on your part will be short lived.