Let parents register once and book for their children

A parent registers once and manages lessons for one or more children, who
need no login of their own. A child is a real wp_users row with the student
role but no usable login — so student_id keeps meaning "a WordPress user"
on every table, and booking, credits, policies and enrolments work unchanged.
A us_guardians link table maps guardian to child.

The signup form gains a parent/guardian tick that reveals a block per child,
with the account-signup questions asked per child rather than per guardian
— they describe the student, not the account holder. Signup policies are
recorded once per child with the guardian as the acceptor, which is the
record that actually means something. A family that half-creates is rolled
back entirely rather than leaving a guardian who cannot re-register.

The booking and enrolment forms gain a "Who is this for?" picker listing
children first, so the default selection is never the parent — booking for
the wrong child is correctable, quietly billing a parent for their kid's
lesson is not. POST /bookings and POST /enrollments take an optional
student_id honoured only for that child's guardian; anything else is a 403.
That check is the authorisation boundary of the feature.

Payments and credits gain a payer: the charge names the child it was for and
the guardian who owes it, so per-child reporting is unchanged while notices,
receipts and the payment step reach the parent. Credit is held by the payer,
so one child's cancellation can settle a sibling's charge, and the daily
billing scan sends a guardian one notice covering every child.

Closes #132

Co-Authored-By: Claude Opus 5 <[email protected]>

src/BlockPreview.php

@@ -169,6 +169,41 @@ class BlockPreview {
 		);
 	}
 
+	/**
+	 * Sample family (manage-children) page: two representative children and the
+	 * add form, with the controls inert so the editor preview cannot post.
+	 */
+	public static function family(): string {
+		$children = '';
+		foreach ( [ 'Ada Lovelace', 'Alan Turing' ] as $name ) {
+			$children .= sprintf(
+				'<li class="us-family-child"><span class="us-family-child-name">%s</span>'
+					. '<span class="us-family-child-actions"><a href="#">%s</a> <button type="button" disabled>%s</button></span></li>',
+				esc_html( $name ),
+				esc_html__( 'Edit', 'unsupervised-schedular' ),
+				esc_html__( 'Remove', 'unsupervised-schedular' )
+			);
+		}
+
+		$add = sprintf(
+			'<h4>%s</h4><p><label for="us-child-name">%s</label><input type="text" id="us-child-name"></p>'
+				. '<p><label for="us-child-dob">%s</label><input type="date" id="us-child-dob"></p>'
+				. '<p><button type="button" disabled>%s</button></p>',
+			esc_html__( 'Add a child', 'unsupervised-schedular' ),
+			esc_html__( 'Name', 'unsupervised-schedular' ),
+			esc_html__( 'Date of birth', 'unsupervised-schedular' ),
+			esc_html__( 'Add child', 'unsupervised-schedular' )
+		);
+
+		return sprintf(
+			'<div class="us-family">%s<h3>%s</h3><ul class="us-family-list">%s</ul><form class="us-family-add">%s</form></div>',
+			self::note( __( 'Editor preview — signed-in guardians see and manage their own children here.', 'unsupervised-schedular' ) ),
+			esc_html__( 'Your family', 'unsupervised-schedular' ),
+			$children,
+			$add
+		);
+	}
+
 	private static function note( string $text ): string {
 		return '<p class="us-editor-note">' . esc_html( $text ) . '</p>';
 	}