Hi Elaine,
I like how you have restricted the children to rafts, so that floating is the key issue, and the rather more complex ideas of buoyancy and boat design can be met when they are a bit older.
In Cork one of the local councillors has run a Model Boat competition for a number of years (14!). This year’s rules are here: https://corkharbourfestival.com/register-now-lord-mayor-kieran-mccarthys-make-a-model-boat-project-2024/
and previous year’s images can be found on his website.
https://youtu.be/JkAAhec70Cw?si=AoLijhjT2DKTw2sd
Have you a large pond nearby that they might use to float their boat?