The key is… (are you ready for it?)… patience. Yep, that’s it. Simple, but not easy.
Be.
Prepared.
To.
Wait.
Once you understand that this may take a while you will not feel the need to rush them or yourself, which will inevitably make this portrait session implode. This applies to photographing children by themselves, with siblings/friends, and in family portraits. In fact, sometimes photographing families with children is even more difficult because the parents get frustrated trying to make their kids behave that it can have the opposite effect. Hopefully your patience will rub off on them.