The basis of accrediting training is the logging of workplace based assessments (WPBAs) demonstrating that at trainee is now competent at whatever skill or ability. Some of these are more obvious tasks than others (eg lumbar puncture vs reflection on communication). A track record of these, combined with supervisors' reports are how the School of Paediatrics and the GMC makes a judgement of whether a trainee is awarded a Certificate of Completion of Training.
Many trainees look carefully to ensure that they have done the minimum number of WPBAs for their training level - and a list of these can be found on the RCPCH website here . However WPBAs can and should be used more expansively to chart what you are doing, expecting that many will function formatively - offering guidance for improvement of that skill. Toward the end of the training year a glance a the RCPCH website checklist will ensure that the minimum number have been achieved. As a rough guide, aim for one WPBA each week.
When you meet at the start of a placement with the ES, think about what you want to get out of that job, but also how you will evidence that in your portfolio.