I always think that outdoor pictures always come out looking much nicer. The photographer is able to pose everyone professionally like he would do in a studio, as well as get pictures that show the kid’s personality and just having fun (think hanging from tree branches, get them to build a sand castle together and get shots of them doing that when they are not looking at the camera, a picture from the back of two or three kids holding hands running on the grass towards a park or on the beach towards the ocean, etc.)
However the best time to do pictures outdoors is right before sunset when the sun is not at its strongest anymore but still there. That is when the lighting will come out the best.