Scheduled Automation to Update a number of child records based on Lookup Field filter

I have part of the Automation already done but run into a roadblock

The Parent is the Event table which holds the Zoom info for the event' one record per Event

The Child is the EOTI Clients and Ticket Creations table which has the checkbox field (Trigger Email Reminder); multiple records may match the 'tomorrow' criteria.

I have the trigger 'daily'

I have the filter (Event Date & Time = 'tomorrow'); Event Date & Time is a Lookup field from the Child to the Event date and time.

I want to 'check' the Trigger Email Reminder checkbox but that field doesn't appear in the Select fields Field dropdown.

Does the automation not have context with the correct EOTI Clients and Tickets Creation records because I used a lookup field?  How do I establish context so all the EOTI Clients and Tickets Creation records matching the Lookup field Event Date & Time get their Trigger Email Reminder checkbox checked?