My big kids wear Crocs and flip-flops just about all summer. Their shoes get a bit nasty.
I filled each of the kids' buckets with hot water and a bit of dish soap. I gave each of them an old toothbrush and asked them to clean their shoes.
They cleaned and cleaned and loved it. They were busy for at least 20 or 30 minutes.
Just a little tip, though. If you don't want them to use the hose to rinse off the soapy bubbles and each other, be prepared to give them an alternate way to rinse the shoes...or supervise them instead of packing for your upcoming trip.