You cannot delete anyone in Family Tree unless you are the sole contributor on the record.
First thing to do is figure out, through the change log, how they got there in the first place. Did someone add them in as children incorrectly? If so, you should message the person and ask they why they though those people belonged in those families. Did some do an incorrect merge of two different families? If so, you sound reverse the merge.
After you know how and why they ended up where they did, then you just go ahead and fix things. Unmerge or Restore if merging got them there. That will put them back with the correct parents.
If they were just directly added to the wrong family, then you delete the relationship using the paper and pencil icon to the right of their name as a child.
This does not delete the profiles, thereby removing other people's work, it just corrects family relationships.
If you post the two ID numbers, more specific guidance can be provided.
If I find someone linked into my family that I definitely know shouldn't be then I unlink them from the family with an explanation why and emphasize my sources. You can do this by going to the relationship button, it looks like a pencil on the family details screen near their name. There is an option to remove or replace relationship.I always try and find out why they added them in the first place.
It is particularly important to check for bad merges.
One of the more common complaints in Family Tree is "Where did my family go?? It was all here yesterday!!!
Picture this: Mr. and Mrs. A and Mr. and Mrs. Z have similar names and live in similar places. Both have lines that go both forward and back with the As having children a, b, and c and the Zs having children x and y.
Someone gets confused and merges Mr. Z into Mr. A and Mrs. Z and Mrs. Z and the single family shows children a, b, c, x, and y. Now x and y look like children of the As and have completely different ancestors than they did before.
If you come along as a descendant of Mr. and Mrs. A, don't recognize x and y, and simply unlink them from the family, then x and y aren't shown as siblings anymore and both have completely lost their families. If one of x or y's descendants come to Family, that person will think all of their carefully done research on the ancestors of the Zs is gone!
If instead you carefully check the Change Log and undo the incorrect merge either through the unmerge function or the restore function, then clean up any residual mess from the merge, then everyone will be back where they belong, all family lines will be restored, and everyone will be happy.
I find unmerging dreadful at times. Sometimes the family I'm not interested in just disappears and I don't remember enough about them to find them again. I must be upsetting someone. I find I have to get out pen and paper and make a plan before unmerging and note everyone's ID#.