Emma,
Space buggies are a perfect example of a design and make that links to the child’s own understanding.
I would strongly suggest including a wheel of some sort, which will allow the children to access the Making skill of “• understand that these materials can be linked in simple ways to allow movement make a wheel and axle using a pencil and thread spools;” from SESE Science, Infants / Skills.
When we have made these with teachers at in-person CPD events, or with children during workshops a range of possibilities open up, with both fixed axle (and rotating wheels) and fixed wheel with rotating axles. Have a supply of straws and skewers and a range of round objects and wheels will happen.