Ducks are small. Everything is a threat to them. And when it doesn't move slowly, it seems even scarier.
How old are your kids?
My friendliest runners were the ones I had the most time to spend with them. They were raised on my patio in August in coastal AL... hot. So I'd bring them inside to our master bath and just sit on the floor and let them run around. I just sat on the floor while they ran around and then over me. Then I became a jungle gym. I just sat. When I got up, I did it slowly.
Then I gave them run space on the deck and again, just would go out and sit with them while they ran around. They'd come and hang out with me when they wanted.
I didn't hand feed, but treats were either in their pool or from my hand.
Head outside with a beach towel and lay down on it and let the ducks run around. Tempt them in close with peas - if they already eat from your hands, peas from your hands. Watermelon is a not so much a from the hand treat - or you end up covered in watermelon juice and a sticky mess.
The more you are on their level and not towering over them, the less of a threat you are. Mine have never been willing to be picked up, but they were friendly, would come when I called (them or the dogs) and would hang out near me if I was in the yard or next to my chair if I was sitting in a chair.
I didn't do that with the 2nd batch of runners. Of those 2, one will eat peas from my hand, but is never in reaching distance of me otherwise.
It really just takes time with runners. If you are just hanging out doing nothing, go hang out and do nothing with the ducks. The closer to the ground you are, the less threatening you are. Treats. Friendly is friendly, doesn't matter if they are only doing it for treats lol