Project Appleseed

I would like to know more about the youngest on the green- William Diamond and John Harrington.

Were their fathers on the Green.
Did they have any siblings or cousins that mustered?

The website lexingtonminutemen.com is a good source for members of the training band. 

Neither one's father was on the field that morning.

Drummer William Diamond does not appear to have had any relatives on the field--or in Lexington.  Diamond was apprenticed as a wheelwright to Thomas Fessenden and in that capacity had lived with Fessenden since 1770, apparently coming to Lexington from Boston.  His gravestone and pension application indicate he was 19 at the time of the fight (rather than the sometimes cited 16).

If you wished to think of Fessenden as standing in loco parentis, he was present--watching from a pasture, and fled (according to his own account)--but not with Parker's company that morning.

There were nine Harringtons on the field, but the father of Fifer Jonathan Harrington III was not.  Jonathan did not have any brothers, but he had two male first cousins (Daniel, the company clerk, and Thaddeus) and six male third cousins (brothers Moses Jr. and Caleb, and their first cousins, brothers John, Jonathan Jr., Moses III, and Thomas.) 

Jonathan Jr. and Caleb, were killed on April 19. Jonathan Jr., famously, died within sight of his home (testimony of the participants did not place him on his doorstep or in his wife's arms, although one said she had seen him fall); Caleb was killed as he attempted to escape the meetinghouse, where he had gone with two or three other men gathering powder or arms.

See the link for bio and a family tree. There are separate bios for each member of the company although the depth and format varies somewhat.

https://lexingtonminutemen.com/jonathan-harrington-fifer/

There is also good information at the following website--and especially cool for a Battle Road trip.

https://www.tourlexington.us/wp-content/uploads/2025/04/Witness-to-1775_FINAL_Links3.pdf

The training band was representative of Lexington, but it was more than a community--it was a family, writ large.  They were more than neighbors; they were intertwined families with ties often through generations. 

One researcher dove into these extended ties with the Munroe family.  There were eight Munroes on the line, but she identified 25 of the 77* men on the field as having family ties to the Munroes--including six of the eight who died.  And that's a floor, not a ceiling.  (*77 was the traditional number used at the time of writing; I believe the company uses 81 now as having "responded" and leaves open the possibility that others might be identified.)

https://nutfieldgenealogy.blogspot.com/2010/04/cousins-at-battle-of-lexington.html

This omits some detail but it is already too long a reply to a straightforward question!  Best regards.  Oz

Thanks, Oz. This will keep me busy for a while!

Good luck!  I hope you're better at answering a question and stopping than I am!