I assume she had just dropped her kid off at day care and was heading to work. Many parents have these trailer bikes. The kid learns balance and can help pedal, or they can just relax.
Most Danish kids learn to ride bikes earlier than in other countries. My son was scooting along the bike lanes, with me right behind, when he was 3 and half with training wheels. Just after he turned four, he learned to ride and we took the training wheels off.
The Danish Traffic Safety Board doesn't recommend that kids under seven ride unaccompanied on the bike lanes.