Do I need to assess all my children at the start of Year 2?

Only if your Year 1 children were not assessed in Summer 2. 

If your assessments all up to date, then you should use your professional judgement and only reassess anyone you are concerned about.

Use the assessments that correspond with the Year 1 term they have most recently been taught. For pupils with larger gaps, refer to the Year 2 flowchart and use the Rapid Catch-up assessment if needed (note that these assessments go on the Rapid Catch-up spreadsheet, not the Assessment tracker).