How to set up the "Child card is blocked" rule?
The "Child card is blocked" business rule will automatically block a parent card if any of its child cards are blocked. To configure and enable the rule for your account, go to the Administration panel at the top right side of your screen and open the Business rule tab.
Select the "Child Card is blocked" rule option.
Note: In case there is a set of defined rules to your account -> use the Add new rule button at the upper right side of the panel.
The "Child Card is Blocked" configuration panel has the following fields and properties (img.1):
(1) Name - give a name of the rule to distinguish it.
(2) Description - provide a description of the rule behavior in the system.
Note: when a business rule is created, it will get a rule ID and will be made available in the Business rule panel under name and description.
(3) Share with: this option allows you to share the rule with other users in the system so that they can edit/disable/enable the rule.
Every business rules have three main control settings: When; And; Then.
It is a sort of "point-and-click" programming language with which you can automate any process. The way it works is based on the programming conditional statement "if this then that". 
(4) WHEN - this is the event that will trigger an action (i.e. When a card is created).
(5) AND - define the child card filter and specify the conditions that trigger the block action.
- When users create a new rule, the Board filter will be applied by default. This means that the rule will be executed only for the boards you specify in the filter. If you remove the board property and enable the rule, it will be executed for the entire account. You can use "or" to add extra boards and optimize the scope of the rule. Use the minus sign to remove a board.
- Click the "Add new property" link to open the Card property match panel and select any of the pills. For example Priority. In the rule configuration panel, you can set it up as Critical. Note: you can select the criteria that will match your scenario.
(6) THEN- if all events and conditions are evaluated as true, the parent card will be blocked.
As a result of this configuration, only child cards located in Board X that have critical priority will trigger the parent block action. 
Important: To save and enable a rule, checkmark the green box at the top right corner of the configuration panel.
Please, read more about parent/child links and how to create them.