Kate Middleton's Kids Play in Her Whimsical 'Back to Nature' Garden

Kate Middleton designed a garden that she hopes will inspire others to get their kids outdoors and playing -- and her kids set a great example by leading the way on that front.

The Duke and Duchess of Cambridge had themselves quite a day exploring Kate's new "Back to Nature" garden that she's officially unveiling to the public in a few days as part of the annual Chelsea Flower Show. The point of all of it ... early childhood development and getting folks (especially kids) reacquainted with the great outdoors. A great cause, really.

Before she opened the doors to everyone else, she had Will bring their own kiddos by to scope the joint out. Prince George, Princess Charlotte and the newest addition, 1-year-old Prince Louis, definitely had their run of the place ... it's practically a kid's playland paradise.

Charlotte and Louis rode the rope swing, which looks super fun ... and Georgie boy dipped his toes in a running stream under a baby bridge. We don't usually say this ... but damn, this is cute.

Seeing how they got a little cousin now who'll be able to play along soon enough -- ya gotta imagine the Royals might be here quite a bit in the near future.