This is a QGIS 3.4+ plugin adding useful actions to enhance relations:

• show children features of a selection of parent features
• batch insert of children features

1. Select some features in the referenced layer.
2. In the layer tree, in the context menu of the layer (or from the attribute table context menu), click on the entry Show referencing features in "{referencing}" for the selected features in "{referenced}".
3. This will open the attribute table of the children features linked to the parent selected features.

It is possible to show children using a custom aggregate (min or max on a chosen field). Go to Plugins -> Actions for Relations -> Define custom aggregates.

1. Select some features in the referenced layer.
2. Switch on the editing of the referencing layer.
3. In the layer tree, in the context menu of the layer (or from the attribute table context menu), click on the entry Add features in {referencing_layer} for the selected features in {referenced_layer}
4. A form shows up to define attributes of the features to be created in the referencing layer (the referencing field(s) will not be shown since they are filled automatically).
5. The plugin will automatically create as many features as there are features selected in the referenced layer. Each of them will point to one of the selected referenced features.