Any child can have a humiliating accident, esp when sleeping away from home. So, before your next child, buy the mattress protector. I have them for the grownups as well, keeps the mattress spotless! A mattress is expensive, a protector only little money. For a peaceful night's rest for you, the host, protect the mattresses your guests sleep on!
For washing duvets, yes to tennis balls (bashes the filling as it goes round). Use half the detergent you would normally use. If there's instructions on the doona, do that.
Stop and take it out periodically, feeling for wet lumps and bash it up again. Even when it all feels perfectly dry, do it a little longer until it is totally fluffy and lump free. Feathers tend to stick together until completely dry. If there's time, hang it for a bit before using it, just to be super shore there's no damp. That is what causes any smell. No need to get rid of them, even if weed on. Relax!