It has been confirmed multiple times in the story afterwards that Traveler defeated Childe in Archon Quest.

Also, the fact that he resorted to Foul Legacy and still failed, simply shows that 2-element Traveler is a stronger than Childe since Traveler had no time limit on their power. Had Childe not ran away and the fight continued, Traveler would just wipe the floor with Childe. On the other hand, Signora is fighting 3-element Traveler.

The only conclusion that can be drawn here is 3-element Traveler > Signora, and 2-element Traveler > Childe.

As for who is stronger between Signora and Childe, that's just pure speculation. Based on the evidence, Signora is more likely to be stronger than Childe, but then again, Signora is a finished product (millenias-old ancient mage turned into living flame) whereas Childe is still growing as a fighter.