An online friend asked me about this the other day.
The novel makes no mention of Bard having a family during the events in the Quest for Erebor. We know he had a son, Bain, and grandson, Brand, eventually, as they were mentioned in LOTR. But there's no record when Bain was born. You can make the case either way, but the fact Bard never checks on his family after Smaug's attack on Lake Town suggests that he was still a bachelor at this point.
People have speculated that Bard was a family man in the films to make him more relatable to the audience. Could there be another reason?