In The Bronze Bow, why does Daniel want to take Joel's warning to Jesus?

The warning that Joel passes on to Daniel is grave: despite their shared interests in rebelling against the Romans, the teachers and Rabbis in the Jewish Temples are upset with the preachings of Jesus. They believe that he is changing and reinterpreting God's word instead of continuing with their own traditions, and because of this perceived heresy, some might be driven to violence. Daniel takes the warning to Jesus, but not because of the warning itself.

With the door shut against him, Daniel stood in the crowded garden. He wanted desperately to see Jesus. He knew now that the warning had been only an excuse. If he could have one word, one sign from Jesus, he might find the strength to go on working.
(Speare, The Bronze Bow, Google Books)

This occurs after Daniel rescues Joel from the Romans, losing Samson and Nathan during the fighting. Daniel is disillusioned and believes that he has failed the cause as a leader; he does not see in himself the strength to continue fighting the Romans. Therefore, his visit to Jesus is not really to warn him about dissent -- Jesus is already aware that his teachings are controversial -- but rather Daniel wants to find new purpose in his own life and struggles. If Daniel can get validation of his trials, he will be better able to weather tragedy and lead his men to victory.

In The Bronze Bow, why does Joel believe Jesus needs a warning?

After Joel's rescue, he returns to his home and finds out that the Rabbis at the Jewish Temples are angry about the preachings of Jesus. They believe that he is changing and altering the laws they received in the Old Testament, and that alteration is against the Word of God. This internal dissent means that Jesus has enemies on both sides, from his own people and from the Roman occupiers. Joel believes that the threat from within might actually be more dangerous than the threat from without.

"I mean the elders of the synagogue. The rabbis and the scribes. They can't understand him. They're furious at the things he says and does. He is too free with the Law. They say he is trying to destroy all the authority of the Temple. Some of them even say he is in league with the devil. [...] Some of them hate him so much -- I think they would kill him if they could."
(Speare, The Bronze Bow, Google Books)

The change between the original teachings and laws in the Old Testament and the new interpretations and sermons of Jesus is one of the most important aspects of the split between Judaism and Christianity. In earlier times, religious disputes were more often cause for violence; Joel is scared that the Rabbis might take the matter into their own hands to remove heresy. Therefore, while Jesus is fully aware of the threat from Rome, Joel believes that he should be cautious even at home.