There's a few problems with the idea that Uther and Jaina could have somehow stopped Arthas.

First off, killing Arthas would have done nothing. There were soldiers there that were loyal to him, and would have defended him. How many innocent men would have died in that fight? Uther and Jaina were saying Arthas didn’t have the right to decide who lives and who dies; how much hypocrisy would there be in their deciding to kill any soldier that would follow Arthas? Even if they’d have managed to kill Arthas, would that have changed anything? The soldiers had their orders; they may well have carried them out even without Arthas. Should Uther and Jaina have killed all of them? (Now, I know the inevitable comparison that’s going to be drawn is with Nazi soldiers following illegal and immoral orders. However, it’s important to note that the Nazi soldiers who were prosecuted were only those that followed orders that no reasonable person could accept as lawful. Arthas’ men don’t fit that criterion; they’d seen men, women and children turn into vicious monsters, and they knew that if Stratholme fell, thousands of innocent people in the surrounding area would die. If it’s possible for us, knowing all the facts, to still debate whether or not Arthas was doing what was necessary, it must be the case that his soldiers could have reasonably believed that they were doing what was necessary.)

So, they couldn’t just kill him. What other options did they have? Jaina had no authority to command anyone. Antonidas sent her as an advisor; she couldn’t order anyone to stand down. The men had just seen Uther stripped of his rank; as Crown Prince, that was totally within Arthas’ power. To have followed Uther would have been treason. Even so, a handful of men DID commit treason by going with Uther; specifically, the other paladins. Now, Uther could have denied Arthas’ authority, and used the handful of men who followed him to try and stop Arthas. However, they’d have been hopelessly outnumbered; he could perhaps make such a sacrifice for himself, but could he really ask that of his men? Ask them to fight and kill their comrades-in-arms, knowing that they couldn’t win, and that the outcome would be the same? No, all Uther could do would be to lead those that would follow him back to regroup and keep them from dishonoring themselves in Stratholme.

So, I think the idea that Uther or Jaina could have done something to stop Arthas is a bit of a stretch; at best, they could have made a pointless symbolic sacrifice that would have cost more innocent lives without changing anything. Instead, they tried to convince as many as would follow them to leave Arthas behind; under the circumstances, I think that was the best they could do.