The park is also named after the family, whose ties to the area date back to the 1930s.
Ugo De Marchi has lived in the suburb since his birth in 1938.
His Italian father had purchased 10 acres there when it was known as Bibra Lake, running a family farm filled with horses, cows, chickens, turkey, pigs, sheep and fruit trees.
In 1955 the State Government took back three acres of land for state housing, before it was renamed Coolbellup.
The remaining seven acres have since been sub-divided.
Mr De Marchi said he was thrilled with the new seat and plaque.
Cockburn Councillor Stephen Pratt pushed for the installation of the plaque, saying it complemented the playground already at the site.