Hi Catherine,
Static Electricity is a great topic for 1st/2nd class (and addresses :Energy and forces >Magnetism and electricity • explore the effects of static electricity) and it is explored in this Curious Minds Junior class resource:
https://www.sfi.ie/site-files/primary-science/media/pdfs/col/static_electricity_activity.pdf
and in Irish: https://www.sfi.ie/site-files/primary-science/media/pdfs/col/static_electricity_activity_irish.pdf
I don’t think you need magnets, and, as you have said, the balloon will stick best on a dry, non-humid day.
I wasn’t too sure what the soft drink can was going to be used for, but found this explanation: https://www.experimentarchive.com/experiments/rolling-static-soda-can/
I’d be careful on the language used, electrons are only mentioned in the teacher’s glossary. Charges themselves are a tricky enough concept. The charges will balance on a humid day more quickly, since water molecules in the atmosphere are able to neutralise charges.