A motorcycle sidecar can be an interesting way to travel -- a rider is close to the ground, and it's safer than hanging on to the back of a cycle's driver. If you want to take a child touring in a sidecar, however, you may want to do a little checking first.
There is no set answer on whether a child can ride in a sidecar -- the laws governing sidecars vary from state to state, with sidecar laws vague and almost nonexistent in some states, but more detailed in others. Some require seat belts, others helmets; some pose little, if anything, in the way of restrictions.
To be sure sidecars are legal, check the state statutes on them. Call the state's Department of Motor Vehicles or locate the statutes online. Do so for every state in which you plan to travel.
Even if it's legal to have a child ride in a sidecar in a state, a little common sense is necessary. A sidecar is no place for a very young child, nor should a child be allowed to ride in one without a seat belt, regardless of what the law says.