the answer depends whether you want to create the children in the front end or in the back end

In the front end, you would probably use a @api.onchange() decorator with your method. In this case, the onchange method is loaded in the front end, and from the children you should be able to access the parent values as they are in a "cache". Practically, there is no difference in the coding.

In the backend, the methods are called on records, so before they are created, you can't access their value... However, if you create them from backend, then you must have their values in a dictionary, so you should be able to get the required values from it.

hope this helps.