@Miss Lydia you were faster! I was about to write the same thing: Do let them play outside when it is warm enough for you to watch them without wearing a heavy jacket. Make sure they get some sunshine and some natural ground under their webbed feet, hopefully there's a spout where your grown-up's have not eaten all the grass. Keep them dry (no outside bathing) and protected from the wind and they will love it - and you too.
Pop out your phone and make a video, you know we all love to watch little ducklings playing…
Exposing them to some colder temperatures will promote their feather growth in the same way as exposure to water triggers their preening gland. I had my spring ducklings out on the patio in February this year!