Hi Cameron, goslings legs sometimes do this and the parents are gigantuan in comparison and step on them sometimes, often resulting in death.
We have had lame ducks and I have wormed them and it's passed in a few days.
Good you are keeping them isolated - they shouldn't move about too much until they strengthen up, you'll probably find a week or so does it. If you want them back with mum though she should be able to see them. Livestock can't always be bothered with the weaker ones unfortunately and just walk off or keep up, but it does give us owners the chance to become closer with the additional responsibility
Hope they all end up healthy and happy, our ducks like brown bread squished to a pulp in ordinary milk when they are teenies and still love it for a treat when grown up