If your children are still young enough to bathe together, your son and daughter shouldn't have a problem sharing a room. However, once children reach the preschool years, opposite sex siblings sharing a room is usually similar to same sex siblings who desire and require privacy and respect.
No federal law bars opposite sex siblings from sharing a room at any age. In fact, the familial status clause of the Fair Housing Act forbids landlords from discriminating against tenants with children or placing unreasonable regulations on them, including specifying how they can share rooms. However, some housing programs might have specific requirements about opposite sex siblings sharing a room. The U.S. Department of Housing and Government, which offers low-income families housing assistance, states that other than very young children, opposite sex children cannot be required to share a room.
Your children's ages are an important factor when considering having them share a room. Once your son or daughter reach puberty, room sharing might become awkward for both of them. According to AskDrSears.com, points out that curiosity about an opposite sex sibling's different body is generally normal, but older siblings might become more uncomfortable with sharing a room then they were as toddlers, preschools or even early school-age children.
Once your son and daughter begin to develop separate interests, whether they follow typical gender roles or not, they will need more privacy. Older children, of course, will require more privacy to change clothes, but even younger children will want to keep certain parts of their life private from their brother or sister. Children might want to keep books, notes from friends, or a diary safe from the prying eyes of a teasing little brother or a nosy big sister, so provide safe space for each sibling.
If your son and daughter do not want to share a room, but space is an issue and there is no choice, considering separating the room in another way. Set up a bookshelf or curtains in the middle of the room to act as a separator. Consider investing in loft beds with a curtain around the bottom part to create a small private area for each child. Small locked boxes will keep personal mementos private, and instilling a healthful respect for privacy in each child will also keep the peace.