If you are doing autosomal testing and you want to help your daughter or son, you should definitely have their father tested rather than one of them, assuming that he is still alive. Any true match for you is going to share a common ancestor with you and your children and there is no need to spend the money to have them tested to know that. Plus since you are higher up the tree you are likely going to have some people on your own match list that wouldn't necessarily show up on the match lists of your children, which is the reason why it is always better to have older generation members tested rather than the youngest ones.

Of course if the children's father is no longer available to be tested, you might want to consider having any of his siblings tested since that sibling would presumably share a good portion of the same DNA from the father's side of the family that your children's father would have carried.