Hi, a very warm welcome in, so you've observed the duration of time that you're able to hold the children's attention span/initial length of concentration for, at which point you've probably seen that the activity itself and the resources you use are better placed to step in, take over their motivation & engagement for a while until you find your input is asked for or you join in to enrich the learning :)
Plan to know what interests the two children, if themes have any part in that eg. cars, dinosaurs, princesses, animals, enjoy using devices eg. handheld tablets and their apps and what physical items you can introduce to bring your phonics work to life and all of the children's senses.
sensory play amazon.co.uk (sadly links to the outdated 2012 eyfs although otherwise has useful ideas)
Then look at any aims or 'targets the teacher is wanting for the children and which books, games, phonics programmes can be used to get you all there.
To help demonstrate your commitment to helping children learn, can you take a look through the class resource cupboards and staff library if possible for books that might help you support children with ideas for enunciation and pronunciation?
50 Fantastic ideas for teaching phonics by Alistair Bryce-Clegg on amazon.co.uk and
the early literacy handbook by Dr Dominic Wyse and Christine Parker
On gov.uk you can download
phonic programme evaulations,
Teaching phonics -
letters and sounds 2007 guidance
Activity ideas and
what are synthetic phonics + phonics made easy from Oxfordowl.co.uk
Hth, best wishes