Hi Nico,
One way to do this is to use a microflow as the data source for your child object forms, within this microflow you can setup the association to the selected parent.
From your description, the selected parent entity will be in context so you can pass this entity as a parameter to the microflow that will create the child entities. You can then replicate this as many times as you need, so create a data view that uses the microflow to return an new correctly associated child entity with the required input widgets and then duplicate this on your page as many times as you need.