We have the people at our check-in desk watch for these things, and we no longer let our check-in volunteers enter new people (because of this problem). We have worked very hard over the past year to ensure that we don't have this happen. Although it is convenient to have them in their household if they bring them and not the parents, it is a nightmare if you try to do a church directory or any other reports. We had one household when we tried to do the directory with 23 kids (they have three). All the rest were visitors.
If the child is not a member of their family, we have them fill out a form that has the child's parent(s) names, address and phone. That Sat./Sun. they get a temporary name tag (we have them pre-made with security codes etc.) Then on Monday when we then enter the new households, the parents are listed as 'parent of child attending' and the child as a visitor. When the child comes with someone that is not a parent, they have to go into their phone number household or by last name to check in. It also helps us to not have just a child listed in our system with no parents. We try to do the same with grandparents, aunts/uncles, etc.
We do have some kids where both parents attend our church but are divorced. In those situations we leave the child in the household of their primary residence.
I find this keeps our data cleaner, and ensures that we have kids where they belong. We have found this helpful in cases of needing to reach parents also regarding issues with the child. There are still some wrong in our system, but we have found this helps with it getting worse. It also makes it easier to clean up data weekly for duplicates.
Good luck.
Donita