Hi Jude,
hmm, this seems a bit unreasonable. Have you assigned them performance management targets for this academic year? As this would be what you should appraise them against.
Otherwise, you can refer to the TA standards and make a reasoned judgement against these. Perhaps if you present it to them in a table and have them self-evaluate, then you rate them also and you discuss the agreed/disagreed ratings in a review meeting.
I’m happy to give more advice if needed.
Kimberley