The singer/actress took to on Sunday (20Mar16) and posted a sweet selfie of the former ice hockey player Mike holding son Luca while Hilary snuggled up to the side of the tot, beaming with pride.

She added the caption, "We are so proud of this boy."

Earlier in the day, she shared a solo shot of little Luca, writing, "You....are magic my little big boy happy 4th birthday my heart just explodes for you..."

The friendly exes spent most of Luca's birthday at a small family gathering, as documented by Hilary's older sister Haylie Duff on Snapchat. The Napoleon Dynamite actress shared photos of her sibling, Mike, and the birthday boy, as well as her husband Matt Rosenberg and baby daughter Ryan enjoying a relaxing afternoon at Haylie and Hilary's mum's house.

The So Yesterday star and Mike split in early 2014, but only finalised their divorce in January (16). The former couple has long maintained the break-up was amicable, even jetting off for a Hawaiian vacation together just days after news of its split became official.

Hilary and Mike share joint custody of Luca, and agreed to split the child-rearing costs in the divorce settlement.

The single mum previously told U.S. talk show host Ellen DeGeneres that she and her ex have remained very close, despite their failed marriage.

"We are good friends and we laugh a lot," she shared, "and we have a great communication and we share pictures when one is not with Luca. He's (Mike) great, we are great and we keep on trucking."