*Season 4 Finale Update
The girl has revealed herself to be Dawn Allen, the daughter of Barry & Iris although whether she follows the same story-line history as the comic version remains to be seen.
We don't know (as of pre-S4 finale), but...
Assuming this is intended to be significant * there are a couple of likely possibilities...
* - Which is likely since Jessica Parker Kennedy (who played the waiter/caterer) also appears in the season 4 "The Elongated Man Rises" episode and is writing in the same symbols as Barry used when he returned from the Speed Force.
Dawn Allen was the daughter of Barry and Iris Allen, conceived in the 30th Century after her father's retirement, but born after his death in the first Crisis. She and her twin brother Don inherited a fraction of his speed. Since they never knew their father, they often felt jealous of their cousin Wally West, to whom Barry was a father figure.
Jenni Ognats (XS)
XS is a speedster and superhero in the 30th Century. She is a member of the Legion of Super-Heroes representing Earthgov in Reboot Legion continuity. Her grandfather is Barry Allen, also known as The Flash, which makes her part of the Flash Family. Her mother is Dawn Allen of the Tornado Twins, her father is the politician Jeven Ognats, and her cousin is Bart Allen.