The text on the yellow, green, and blue string pack reads, top to bottom:
"Duncan's
Genuine Gold Seal
Official Egyptian Fibre
Yo-Yo
Top
Strings
If it isn't a Duncan, it isn't a yo-yo
The Original Yo-Yo String
2 strings for 5¢"
The boy is wearing a sleeveless sweater with a "Champion Duncan Yo-Yo Winner" patch sewn on the front.
These string packs were retailed from about the mid 1930s to early 1950s.