No, it is not a prerequisite for most position in the major cities, however if you are placed in the country it would be useful. You can expect your own room as an au-pair, provided within the family home. Homes in the cities tend to be smaller than homes in the country.
Belgian families require very little domestic assistance from their aupairs.
You will be required to contract for a minimum period of one year. Au-pair pocket money is controlled at BEF12000.00 per month. JCR will ask you to contract with your host family for one year. You can only go once to Belgium on an aupair visa and if you have had a prior work permit Belgium will not accept you. They are set by the Belgian Government and unless you are within the age limits requested, you will be refused an au-pair permit for Belgium.