Magpies aren't wild ducks they are domestic even if you found them in the "wild". Someone dumped a domestic duck at some point in time. I may be a bit critical since they are not receiving a complete feed and you asked for advice. I gave advice regarding grit and then you mentioned what you are feeding. But,you didn't want to hear the advice regarding a complete feed because you have your mind set. It's a response I see often. I have no recommendations for you, but where is the research that you have done to feed wheat, peas, and mealworms - which from your other post you mentioned the wheat isn't even ready?
I posted helpful links - maybe give the duck forum a glance and read the sticky posted regarding ducklings.
I'm critiquing your feed choices that is true. It happens on BYC. Lots of people come for advice and want pampering. That doesn't happen here which is something I admire. But, like I said, if you show me your research and that you are feeding the ducklings a complete feed, I'd gladly back your choice.