Enforce that all "Enforce expression constraints" of child tables are valid before I can save the parent feature

Question

In QGIS 3.16 I set up a relation between two tables.

In the child-table I set a constraint-expression and checked "Enforce expression constraints".

If I add one or more features to the child-table which have attributes, which are not valid, QGIS recognizes them as "invalid" as expected but:

1. I can still toggle editing mode for child layer - although some data in child-layer is "invalid"
2. I can still save the parent-layer ("OK"-Button is not disabled) - although some data in child-layer is "invalid"

How can I prevent the user from saving invalid data in both cases without python-scripts (like How can i prevent a form from closing unless data is accepted by validation) ? (I thought to set up a constraint-expression and "Enforce expression constraints" in the parent-table, which checks all child-feature data - but there seem not to be a way to do this.)

I'd like not to use Python because:

1. I have a parent-/child-relation (which is -as far as I know - very complicated to realize with custom forms) and
2. I'd like to use this project in QFIELD, which has - as far as I know - limitations with custom-forms - so I use drag and drop-forms