At 6 years old the family will have support people in place that can offer information. As well there is a down syndrome society that has excellent info. The parents may also qualify for additional money for services such as childcare and that could top up your fees.
Make sure you have a written contract that spells out what time child will come, go home, what is to be provided during the day. For a fee I charge a daily rate of $50 day on the weekend when I do it which is $5 an hour for a 10 hour day so not far off what you are charging and rates here for the week are $35-40 day.
If behaviour is an issue then be sure you talk with the family about what kind of issues there are, how they handle them and what the consequenses are at home. Generally children with Down Syndrome can be the sweetest, loving want to please type of children BUT if they get upset or something in the environment bothers them they do not know how to handle it and will go to extremes of behaviour. Once you know the child's quirks - what he likes, doesn't like, what not to do etc. then it can be very rewarding. Expect his understanding and abilities to be at least slightly below that of what you would expect for his age so you need to watch how you word things or how complicated your instructions are, also that you give him enough time to comply as the processing of the request can take longer.