Although the multiple insert elements are dynamically named, their names are kown. The first one is Fieldname_0, the second is Fieldname_1 and so on. So I believe it must be possible in some way to validate that Fieldname_0 must be required using javascript, but the client side validation tool that was a part of Validation Toolkit and CSS Form Builder is not a part of Data Bridge as far as I can see.
Regards
Rune