If there are wild ducks living there already, and the pond is public property, why would you want to introduce domesticated ones of a different species? Either they would be driven out by the resident mallards, or they would crossbreed and produce the funny-looking mongrel offspring we get on the river Test here in Whitchurch. Your relatives would have no control over them, they couldn't look after them separately from the wild ducks, or even catch them once released on to the pond. Have they got any experience with keeping ducks, and have you actually discussed this with them?
It's a generous idea, but I don't see the point, really, if they're interested in relating to the existing ducks, why not buy them a big bag of proper duck food, to feed over the winter, instead of the awful white bread I see people throwing to the ducks here?