
Maybe you could reassure them by putting specifically what the requirements are for lifting the endorsement in the contract, for example:-
1. Age of bitch
2. Exact scores/results for what health tests under what schemes
3. Specific placing in what type of show.
That way it is crystal clear what is required and they would have something in writing to take to the KC if you renegued on the agreement (which I'm sure you wouldn't :))
If they were not happy with your requirements you probably don't want to let them have the puppy anyway.