The 'Alias' actress and Affleck hosted the celebration at Kidville in Brentwood, California, reported Us magazine.
"The party had a jumpin' gym theme with face painters, balloon artists and glimmer tattoos," a source said.
"They also did a sand art project and had gym time which included freeze dance, a jumpy castle, zip line and a medal ceremony," it added.
Seraphina is said to have dressed up in a pink princess dress and was gifted with a bright orange birthday cake.
The couple are also parents to 6-year-old Violet and 10-month-old Samuel.
Garner recently said that she doesn't want any more children.