Arlen's note is a paper note in Fallout 4.
Location
It can be found in a random location, next to the Buttercup toy, if the player character chooses to give Arlen Glass the toy parts in the Giddyup 'n Go quest. They may find the completed toy along with Arlen's note several days later. One of the possible locations is just south of the Boston mayoral shelter.
Transcript
For a child who needs it.
- Arlen Glass