The ornamental design for a strap for a watch-shaped toy, as shown and described.
FIG. 1 is a front perspective view of a strap for a watch-shaped toy;
FIG. 2 is a front view thereof;
FIG. 3 is a further front perspective view thereof, with the buckle and loop removed for clarity; and,
FIG. 4 is a further front view thereof, with the buckle and loop removed.
The broken line showing of the watch and strap is included for the purpose of illustrating the environmental structure and forms no part of the claimed design.