I've used this facility before. Today I had an interesting situation.
I had several contacts listed in the Child Items pane whom I wanted to put in the same "City". Some of the contacts were already in that city and the others weren't assigned any city. I selected ALL the child items and tried to insert the "City" attribute from the Item Attributes pane. However, the "City" attribute wasn't available. With experimentation I found that if I carefully selected only the items that didn't have a "City" assigned, I was then allowed to add the attribute. In other words, with multiple items selected, you cannot add an attribute unless no items in the selection have a value assigned to that attribute.
Kinook, is there a good reason for this or is it a bug?

If all of the selected items already have a City attribute (whether their values are empty or not), you won't be able to add it, but City will be displayed in the Item Attributes pane, allowing you to edit the value for all items.

If some of the selected items have a City attribute and some don't, then by design, the Item Attributes won't show that attribute and it can't be inserted (otherwise it would update any items already containing the City attribute to NULL when inserted).

Perform an advanced search on (Item) is Contact and City does not exist, select all the search results, and insert the City attribute. Then you can select multiple contacts as above and edit their City attribute at once.

Also, I'm not sure how a Contact could end up without a City attribute (since it's not deleteable), so maybe I misread the question.

Thanks for you response Kevin and well spotted about the potentially "odd" City attribute behaviour. I replaced my actual problem terms with Contact and City - sorry 'bout that. ;)