Its what Edward would do. Picking a younger son in Real Life would often lead to Civil War, since it was the right of the eldest to succeed, despite his abilities.
And the very same Edward then tell his oldest son... Go with 4 groups of English Knights, 5 longbowmen, and 6 armoured sergeants (looks like a decent army)... give a punch to William Wallace... but remember son, to show your bravery and worth,... you must lead the charge right onto scot's william wallace's personally... when wallace was move in front of their pike formation... and edward secretly order some of his knights to stop... letting his son die "Heroically - with killing Willam Wallace"

That sounds a nice roleplay, since the other nobility will perceive that his firstborn son was a real hero...