What's the total weight of this trailer plus kids and other items inside? The reason you run 100 psi with narrow bike tires (or 60 psi with 1.75" tires) is because you've got a total load of around 175 lbs or so and need that much pressure to avoid pinch flats. Trailers are usually much more lightly loaded and should be run with less pressure. Otherwise they'll have a harsh ride and the tires will tend to bounce when hitting bumps making the ride even worse.

For best efficiency get some tires with minimal tread pattern (slicks should be fine) and a soft, compliant sidewall so little energy is lost when it flexes (it'll also give a smoother ride to the kids). If they have those characteristics the width won't matter much and you may as well use some fairly wide ones so they'll ride more smoothly over rough pavement areas.