Not sure I fully understand. You left 2 x children less than 9 years old alone in the room? I find it weird the hotel would come to the room to check who was registered. It is law in some of Europe that every guest needs to be registered. Did you register all the guests at reception? Something seems weird. Maybe something triggered concern for the children alone in the room along with short notice booking of the room?

Also sounds like they said “ok” to you leaving, at no point did you ask specifically and have agreement about a refund?

Marriott customer care simply open a case in their CRM (Salesforce) and pass it over to the property to respond / close the case down. Most Marriott hotels are franchised. Marriott don’t care you booked 2 hotels in the network.