Fans of The Flash are likely still processing all the crazy that went down during "Crisis On Earth-X" with Supergirl and the rest of the Arrow-verse. That said, one of the biggest Easter eggs was kind of a small moment in the epic tale. The various clues Dawn Allen is the waitress at Barry and Iris' Flash wedding seem to prove the Supergirl episode introduced a major new player for The Flash — Barry and Iris' future daughter.
Before the wedding gets started, this mystery girl approaches Barry and offers him sparkling water. In the comics, Dawn and her twin brother Don never knew Barry as a father growing up. They're born in the 30th Century after he dies, and grow up jealous of the relationship that Wally West had with him in life. Both siblings have the speed force, and while they are initially taught to hide their powers, Dawn and Don ultimately become a crime fighting duo called the Tornado Twins.
There are a few other characters/relations that this mysteriously eager cater waiter could be. There's Iris West II, aka Impulse, who's a speedster and actually the daughter of Wally and Linda Park. There's also Jenny Ognats, a speedster called XS, who is Dawn Allen's daughter. The Flash is and always has been a story about families, and the West-Allen family is ever expanding in the comics and on the CW series.
All that set aside, Dawn seems to be the more likely candidate for the waitress' identity. Here's why.
1. She Has A Family Resemblance
She has Iris' smile and Barry' awkwardness. As far as casting goes, she looks enough like she could be their biological child. Speaking of...
2. She's Kind Of A Big Deal
Actor Jessica Parker Kennedy is too established as a performer to be a glorified extra in an episode of Supergirl. You may know her from playing heavily supporting roles to starring in shows like The Secret Circle, 90210, Colony, I Love Bekka & Lucy, and recently Black Sails. She even had an arc on Smallville, and Supergirl in particular loves to pull actors with DC on their resume into the fray. There's no way that her story is over in the multiverse.
On social media, Kennedy has not so much as mentioned working on Supergirl — which does make it seem like her appearance was a huge secret and a more official casting announcement is pending.
3. She Totally Knows Him
If Barry saying "have we met" isn't enough to get you thinking, the fact that this young woman is clearly lying about rhe-ecognizing him and saying that the wedding will be "one for the ages" is definitely suspicious. That indicates time travel and a connection. Come on out, Dawn.
4. She Cares About The "I Do"
Why Dawn would have traveled back in time to witness, or more likely ensure, her parent's wedding is still unknown. All we did glimmer in that scene is that the waitress is invested in the wedding actually going down. Something, like perhaps her own existence, is on the line.
5. The Canon Concludes
Dawn and Don show up in the comics after the first Crisis On Infinite Earths. The fact that this waitress appeared during "Crisis On Earth-X" could therefore be a clue.
6. The Twins Have Been Teased
In the Season 4 premiere, after returning from the Speed Force a space and time-addled Barry says "we're gonna need more diapers," which is exactly the kind of crazed joke you make when you find out you're having twins.
7. The Crossover Effect
Oliver Queen "bumped" into his son on an episode of The Flash, so it seems natural that Barry would meet his daughter on Supergirl. Does this mean that, if she is Dawn, the Tornado Twin will become a hero in Kara's world? It was technically Kara's show at the moment, though the crossover event didn't exactly work like that and the wedding took place on Earth-1. She'll probably reappear on The Flash.
If it is true, just the fact that Dawn is time traveling to meet her Dad and risking a break in the space-time continuum should be enough to prove she is Barry Allen's daughter. But hopefully the show will provide even more concrete evidence soon.