I had a little Spanish Game hen hatch a goose egg one time. The gosling was as big as mom in a month and I had them penned in a 30" X 30" maternity pen. When I moved them to a larger 4' X 5' pen, mom wanted to roost, but baby couldn't He set up a squall that had me concerned about predators, so I put them back in the small pen every night for about a week. When he finally had most of his feathers, I put the hen back with a cock and baby was beside himself. he wanted nothing to do with geese, because in his pea brain, he was a chicken. Took about a month for him to accept the fact and begin to socialize with the geese.
Put them under her at night, so she wakes up with them in the morning. Be sure to be there when she wakes in case she rejects them. I would caution you to keep them away from the pond, of possible as a hen may not be able to provide enough oil for their down to keep them as boyant as they need to be. Hopefully all will go well, Good luck.........Pop