Not having your own kids puts you at a huge disadvantage when it comes to dealing with them.
When every we get a family in the studio we shoot the kids individually FIRST. I’ve never not managed to get a half decent shot this way.
Then if the kids won’t sit still or all look in the same direction, at least you do have a good shot of each of them. And if you’re clever you can sell them 3 prints in a big frame. (Always have one handy to show the parents).
But don’t forget that everyone has a half decent camera these days so the chances are that they are coming to you because they can’t get a good shot of all three. In other words – it’s a hopeless case!