Shulagh,
this will require some careful manipulation to stop a dripping hand getting the interior object wet. You might want to tie a string around the wrapped toy and dunk it in the water, think of your ‘biscuit dunking’ technique. (which could make a fun take the next step, see this now closed competition: https://community.stem.org.uk/blogs/tim-bradbury1/2024/01/17/student-competition)
Would you have the toy coated with something that would show clearly if it got wet? Depending on the toy, a coating of custard powder might be effective, since it changes to a bright orange once wet.