Hi!
Apologies for the late answer. I posted a warning on Unity Forums, that this week I will be slow to support because of special circumstances.
First of all, all static methods that accept a "target" filter don't work on a gameObject basis, but require the true target you used. For example:
// A transform tween
myTransform.DOMoveX(2, 1);
// is killed like this
myTransform.DOKill();
// or like this
DOTween.Kill(myTransform);
// but not like this
DOTween.Kill(myTransform.gameObject);
To kill all tweens on all children your approach is correct (you can scrap the gameObject filter), but it will kill only the tweens that have the main transform or a child transform as a target, and not, for example, tweens that are active on a material. If you still have a callback being fired, it means that you must have some other non-transform tween firing it (or maybe a tween with the same callback that has a completely different transform as a target?). If you're unsure, you could always give an ID to all the tweens you create on a gameObject (and just so you know, you can also use the gameObject itself as an ID), like this:
transform.DOMoveX(2, 1).SetId(transform.gameObject);
and then you can do:
DOTween.Kill([font=Verdana][size=2px]transform.gameObject[/size][/font]);
Cheers!