I’m doing these knee tucks on the Hanger 44 Suspension system. A great way to vary them is by alternating pulling your knees in to the right and then left. This can also be done on a stability ball but is not for beginners as a beginner’s back may not be supported by a strong base of abdominal strength just yet.
Remember that there is always a progression with exercise from easier movements to more challenging movements. This is the safest way to avoid injury. Your trainer should have an instinct based on extensive experience and a proper education (preferably a 4 year degree) in exercise prescription.