If the traineeship takes place in another educational establishment, you must attach a certificate of enrolment from the original university and an attendance certificate from the host establishment, dated after the start of the course (do not attach the letter of acceptance, which does not mean that your child is actually there).

If the traineeship consists of practical experience (e.g. language teaching assistant, work experience in industry, etc.) and is remunerated, you must attach the contract/agreement if any and/or a proof of income.  Your case-handler will study the documents provided to decide whether your child can still be regarded as dependent.