Lucy,
I like how you have planned to engage very young children with day and night and look at the observable features of day and night to start with.
You could add in Earth images that show the Earth during day and night – here is a lovely one from ESA
source: this tweet from ESA
You can get current images from Met Éireann here: https://www.met.ie/latest-reports/satellites/world-visible or direct from EUMETSAT here: https://www.eumetsat.int/real-time-imagery/earth-view
and a video from ESA is here:
https://www.esa.int/ESA_Multimedia/Videos/2013/12/From_night_to_day_to_night_again