Using filter criteria from two different fact tables.

So I think I've solved it.

Just for the record: There should be just one homecoming date per child.

To be able to compare the two dates from the two fact tables I created two calculated columns in DimChild.
First one like this.

HomecommingDate =

    CALCULATE(

        MIN('F_Homecomming'[ActivityDate].[Date]);

    )

The other one (because I wasn't able to solve it in the first step) like this.

HomecommingDate_Plus14Days =

    DATEADD('D_Child'[HomecommingDate].[Date];14;DAY)

Then I used this calculated column in the comparsion when creating the measure.

CALCULATE(

                           DISTINCTCOUNT('F_Visit'[D_Child_dKey])

                           ;...

                           ;'...

                           ;FILTER('F_Visit';

                                  'F_Visit'[VisitDate] <= RELATED('D_Child'[HomecommingDate_Plus14Days])

                           )

)

Feel free to comment if there is a better way of doing it...

Cheers