You need them to practice in a focused and meaningful manner. (real practicing not messing about with stand / bow tightening / adjusting shoulderrest!!) every single day (little and often) not one big session the night before lesson / exam!!!
try and do scales and arpeggios every day... you call out "g major" "d major arpeggio" etc. Look up the syllabus and use the technical terms that the examiner is going to be using, so they are used to the vocab and them being muddled up and not in one set order.
Pieces... Not just from the beginning to the end, but focusing on the tricky bits first, then the whole piece...
thinking not just about accurate notes (in tune and in time) but also dynamics.
very much needs to be supported practice for little ones. Lots of encouragement and praise.
Am lucky as I play too, so helps me supervise my Childs practise. Working towards grade 1 and we do 20mins together every day including singing / clapping rhythms, playing pieces, scales / arpeggios, sight reading.
Left to their own 'practise' I think a lot of young children struggle to know what to do and will play the pieces through and that's about it.
Most music teacher will write in a practise notebook for them, and you can read this to them and remind them of the practise points before they start each piece (e.g. Use long bows, don't forget to count the rests etc)
Musical talent is usually 1% inate musical talent and 99% practice / dedication! The more quality practice they put in, the more they'll get out of it!