Hi, you'll need to use the policy and procedure that your setting has in place for these situations as you'll be expected to collect particular information and record it in a book that parents might then sign. You probably wouldn't want to suggest how a child had hurt themselves as this leads the conversation and can avoid a full expalnation being given to you. It's a sensitive issue, and one that you'll feel far more confident about once you've asked your colleagues/management about the type of language they use at these times.
Page 96 of this EYE handbook looks at safeguarding policy and procedure, and to understand how to respond to evidence a child has been abused or harmed.