It's not whether it would be successful, but rather a matter of should mama and babies be messed with in that way... I completely understand wanting tto i show your son a duckling hatching, but there is risk of mama not taking it back, possibly injuring/killing it, abandoning it, or causing her to abandon the whole brood... not saying these things will happen, but they are possibilities... are you bonded with your runner? Does she trust you? If she does and will let you handle her babies, then you might be successful... the other thing is, if she won't take it back then you're stuck with brooding and caring for a single duckling until it can join the others outside... plus, you would have to integrate it back in with the flock... not to mention that lone ducklings are extremely needy and require much more care and time than a group... along with making sure it knows it's a duck as it grows up...
Best thing to do, if you want to show your son a hatching duck, then I would plan a set of duck eggs in an incubator just so he can see it... brood, sell or give away the ducklings afterwards... this way there is no way to cause any possible trauma to the broody mama or her babies...