If you are using nested labels and want to find all messages that belongs to one label and to the contained labels then search for
label:(Parent), then select all messages and mark them as read.
Assume that the structure of nested labels are as follow:
1 Search for
label:(Parent). This will return all the messages that belongs to
2 Click the Select: All link above your messages.
3 Click the link that says Select all [number] conversations in [current view].
4 Select the action you'd like to take from the More Actions... drop-down menu.
The label search operator only search for whole label names. If the label name include special characters or has misspelled, in order to work, should be written exactly as the label is.
Advanced search - Gmail Help
Automatically mark or move multiple messages - Gmail Help