Ben Affleck says his ex-wife Jennifer Garner was ''devastated'' that their son didn't like the costume she wore for his birthday party.

The 46-year-old actor - who has children Violet 13, Seraphina, 10 and Samuel, seven with the 'Camping' actress - has revealed that his former spouse ''felt so bad'' after she got into the spirit of the 'How To Train Your Dragon'-themed bash and the youngster wasn't very impressed.

Jennifer dressed up as Astrid from the animated movie, complete with blue and orange face paint, a fur shawl and boots, arm sleeves and a pointy, leather skirt, and even Ben was surprised by her costume.

Speaking to 'Entertainment Tonight', he said: ''I was shocked as anybody when I saw Jen come out as Astrid. But then she said Sam was like, 'OK Mom. We already have an Astrid.'

''I felt so bad. She was devastated. She had gone to all these lengths to put on the costume. We're just not that cool.''

The former 'Alias' star had previously admitted the party had made her realise her son no longer thinks she is ''cool''.

She shared a photo of herself presenting her son with his chocolate birthday cake and captioned the Instagram post: ''Well, guess what. It turns out 7 is the age my kid stops thinking it's cool when I dress up for the party. #youngestchild #firsteyeroll #anyoneneedanAstrid [facepalm, laughing, grandma emojis] (sic)''

But while Samuel wasn't impressed, America Ferrera, who voices Astrid, took to her own Instagram account to praise the 'Camping' actress for her efforts.

She shared Jennifer's original post on her Instagram Story and wrote over the top: ''Ahhhh!!!! Yeeeeessss!!! AMAZING @jennifer.garner as ASTRID!! (sic)''