I noticed that sunflowers are used at another point in this episode: when Li and Sakura are doing classroom chores, Sakura can be seen putting sunflowers in a vase. A quick look around the Web for a Japanese symbol meaning of sunflowers didn't give much, but condsidering they are Kero's favourite flowers, they should be attributed much the same way as in the West: strenght, warmth, etc. Considering the cheery little melody that was played at the shot in question, I'd say it was just a random pick, signifying Sakura's (and, assumedly, the audience's) happiness at Syaoran's announcement.