When there are kids, there's fun!
Therefore, my biggest advice to you, dear parents, is don't stress out if your kids won't smile or look neat in the picture.
Let them be themselves (sticking out their tongues, picking their noses, getting grumpy, moaning..etc) and just have fun with them!
Trust me, one day you will actually cherish those bloopers, those "unperfect" frames which, captured your kids' character and spirit, even more than posed frame-ready pictures!