Firstly, why specify 'erect penis' as though that's where the line is crossed? Does the question only apply to parents of girls, since they're less likely to have seen a penis before?

But yeah, I wouldn't mind all that much; it would raise a red flag for sure, given that many grooming operations work by starting out as raising children's curiosity, but I do not see any inherent issue with my child seeing a penis (erect or not!).