To sign accident, as in, "Oops! Something went wrong," create a letter Y sign with your dominant hand, then hold it up to your chin, palm facing you.
To sign accident, as in a car or other vehicular accident, create a modified number 3 sign in ASL on both hands, where your thumb, index, and middle fingers are sticking out. Hold each '3' hand sideways, and make both hands approach each other at the center, as if there are two cards colliding. As the two "cars" collide, bend the index and middle fingers, suggesting car dents.
Teach your child the accident sign in relation to a mishap or medical emergency that you see in a story book, movie, or in real life (though this last one will hopefully never be the case).
While this hopefully never happens in your toddler's presence, teaching this flash card in conjunction with other medical or emergency-related signs can be helpful in the future.