Thanks everybody for looking. With regards to the rules, no money is being given to charities or asked for. All we are trying to do is to take some kids flying for 30 minutes or so. Therefore the only thing being donated is time. Thanks for the info though, it is well worth pointing out.
There's an argument that waiving landing fees for those taking part is valuable consideration. IMHO, it would be a very petty attitude to take, but given the Kenyon prosecution recently, you may want to square it with the CAA first. One possibility might be to waive
all landing fees that day, which would break any contractual link between participation and the freebie.
Good luck with what sounds like an excellent project.