Kate Middleton Wishes Prince William A Happy Birthday With Sweet Pic

Kate Middleton is shifting the spotlight off her health issues and onto her hubby Prince William -- who turned 42 today ... and who got a helluva shout-out from KM.

The Princess of Wales marked the special occasion by dropping a photo on X Friday, showing Wills and the kids -- Prince George, Princess Charlotte, and Prince Louis -- rocking casual gear and jumping for joy at a beach.

She captioned her post with this ... "Happy birthday Papa, we all love you so much! Cx." From the looks of it, Kate might've snapped this at UK's Norfolk Beach ... where Wills and the kids were rocking the same outfits in a similar shot for Father's Day just a few days ago.

Kate, ever the aspiring photographer, snapped both pics, which is why she’s missing from both of them.

It's certainly some light-hearted fun for the royal family ... who’ve been in a rough patch lately with Kate’s cancer battle -- announced in March via an emotional video. Of course, King Charles is also fighting cancer.

Still, it seems things are looking up. Last Saturday, Kate made her grand return to the public eye at London's Trooping the Colour Ceremony. Even Charles swung by to soak in the pageantry ... so it was a good sign the royals are doing relatively well amid hardship.

In fact, sources with direct knowledge recently told TMZ Kate's treatments have gone well over the last 3 months, paving the way for her recent outing.