So, your kids won't look at you anymore.

That's tough. Especially because this picture you are *trying* to take is for the annual Christmas card. Baby is crying and cold, toddler is getting into everything BUT picture taking. Here is exactly what you do: You go with it. Instead of making your voice hoarse and your nerves snap, just go with it. It's not going to work every time but I bet it does 99%.
Be sure to have someone with you who can stand behind you and be a distraction. Sometimes the kid will like to look at them instead of you with your camera, especially if your accomplice can make funny faces. It's far more entertaining for them than "Will! Hey Will! Smile! Look over here, smile dude. Why won't you smile?"
Another trick is to tell them to look for something that isn't there. For instance "Hey Keely, who is behind momma?" They look, you snap. Probably not going to get a smile but perhaps she shows her natural beauty which is plenty for the pic. It could even be better, sometimes smiles are cheesy and a serious shot is more captivating.
Finally and lastly, promise them something if they look. It could be a treat, food or a peek at the picture. If they know they are going to instantly see it kids often give a good look for the picture. Whatever look they give is going to have personality in it and that's what you want to ultimately achieve.
Remember to not stress out about it. Kids read into that and feed off of it. If its just not working, drop it. Go play. Go eat. Go somewhere else. Try it again after a few. Or, hire me.