Prince Louis Joins William & Kate, Siblings in First Public Royal Easter

Prince Louis has crossed a Royal threshold -- going to church on Easter with his family ... something his older siblings didn't do publicly until just last year, so he's got 'em beat.

The youngest child of Prince William and Kate Middleton -- who's 4, but turning 5 later this month -- was seen out with the whole brood Sunday as the Wales crew made their way to mass to celebrate the holy day ... with everyone color coordinating in crisp, blue outfits.

Louis looked well-behaved as he held onto his mom and dads' hands while coming to and leaving St. George's Chapel. He even shook hands with the priest ... which was pretty cute to see.

It's interesting that he made his big debut at such a young age -- you see, Prince George and Princess Charlotte only got out there for the first time last year at ages 8 and 6, respectively. Despite PL's history of acting out, it would seem Will and Kate trusted him.

From the looks of it, Lou put on his big-boy pants shorts and kept it together through what is undoubtedly a boring holiday for little ones ... at least when an egg hunt's not in play.