Making a centaur costume for a child is an unusual costume idea that many children will love, particularly if they're becoming interested in mythology. Making the costume is somewhat time-consuming, but the costume looks fairly realistic when finished. To disguise the area where the costume attaches to the child, you can have the child wear a regular shirt over the costume, or have him wear a muscle shirt to create the bare chest look that most centaurs have.
Have the child stand in the middle of a room with enough space around him to build the costume. Cut two cardboard "legs" to equal the same height as the top of the child's knees. Use duct tape to hold the legs together.
Cut a large rectangle from cardboard to make the body of the horse. Curve the cardboard to make a rounded shape. Have the body reach from the base of the child's back down to just below the child's rear end. Keep the front half of the body flat, but round the back end like a horse's actual body.
Tape the legs to the body and add any additional tape or cardboard to make a semirealistic horse shape. Tape two wheels to the cardboard legs so that the child can draw the costume behind her.
Cover the body of the horse with stuffing to make a soft body. You can create additional shaping with the stuffing. Tape the stuffing in place with duct tape.
Have the child put on the trousers. Place the horse body directly behind the child. Attach a pair of suspenders to the front of the child's trousers. Attach the back of the suspenders to the front of the horse body. This will make it easier for the child to pull the costume.
Cover the body of the horse with the suede fabric. Cut the fabric to fit as you go along, trying to make as few seams as possible. Glue the fabric in place with a hot glue gun. Cover the trousers with the fabric as well, so that the trousers become part of the horse's body.
Cover the wheels with black fabric to look like hooves. Glue a costume wig to the back end of the costume to make a horse tail.