So, you've got to get your mare and foal to a neighbor's farm, a local show or a sale? With some planning and patience, you can make your youngster's first trailer ride an unremarkable, and therefore good, experience.
Arrange your trailer to make a room for mother and baby. In a two-horse trailer, remove the partition to create a box stall effect, and secure a piece of plywood against the breast bar so the foal can't scoot under it. For short trips, very small foals can even be put in front of the breast bar, under the mare's chin; just be sure to secure the front-door handles. Four-horse trailers usually have an optional box stall setup, while unpartitioned stock trailers need no alteration.
Even if you put leg wraps on the mare, don't bother with wrapping baby. Cumbersome bandages on tiny legs pose more of a hazard than a help, and mare are amazingly aware of where they can put their feet without harming their foals.
When it's to load up, park the trailer in a quiet, enclosed area, such as a paddock, and lead the mare in. Younger foals almost always follow along without the slightest hesitation, but older ones may be harder to convince. The closer they are to weaning age, the more suspicious they're likely to be. A six-month-old foal running around loose outside while his frantic mother squalls in the trailer may eventually decide to board, but somebody's nerves will be well frayed in the process. Better to have the several-month-old foal already haltered and under the control of well-established leading habits, so you can help him through the rite of passage with dispatch and minimal upset.
Once the pair is in the trailer, the foal, even an older one you might have led on, should be left loose. Foals in transit usually carry on with their routines of nursing and resting, and will do so safely if given enough room. Tie the mare with enough rope to let her balance and eat her hay comfortably, but not so much that the baby can get entangled.
Six hours in a conventional trailer is about as long as a foal should at a stretch. After that, give the travelers a two-to three-hour break to prevent fatigue from taking its toll on the tiny body. As for your driving technique, make all your turns, accelerations, decelerations and stops slow and steady, for the sake of your precious cargo.