add required pmpro_asterisk to child fields username/email/password

[kimwhite]

When adding Child fields at registration added class and span to show required.

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[dparker1005]

We may also need to update these lines so that fields generated by PHP also show the asterisk: https://github.com/strangerstudios/pmpro-sponsored-members/blob/c03ace06530fa0c948fd4257b249e6ecb9ae5f76/pmpro-sponsored-members.php#L1064-L1085

[dparker1005]

Note that this PR will still not cause fields to turn red when left empty like our billing fields do. Implementing this would require adding the fields to the global $pmpro_error_fields array, which adds the 'pmpro_error' class here: https://github.com/strangerstudios/paid-memberships-pro/blob/49222e4f5e5bce891c59228737d8ea80ae01d254/includes/functions.php#L2657-L2659