A compact costume repair kit can solve small problems before they interrupt an event. Useful basics include safety pins, a few sewing needles, neutral thread, small scissors, spare buttons, and a fabric-friendly adhesive product if appropriate for the materials. Add clear tape for temporary non-fabric fixes and a few hair ties or elastic bands. Choose contents based on the costume rather than carrying a large collection of tools you are unlikely to use.
Test any adhesive, tape, or fastener on a hidden area before applying it to a visible surface. A safety pin can close a small gap, but place it so the point cannot reach the skin. For a loose button, a few quick stitches may be more secure than a temporary fix. Keep sharp tools covered and store the kit in a closed pouch, especially if children will have access to the costume.
Before leaving home, check seams, straps, closures, and the attachment points for accessories. Bring a small amount of matching thread or a spare fastener when a particular component looks vulnerable. Ask someone else to check the back of the outfit, where problems can be hard to notice. After the event, make permanent repairs before putting the costume away. That way, next time's kit remains a backup rather than becoming part of every outfit.