We just lost our not-so-old cat to heart failure, and had to explain it to our 2.5 year old girl, too. We picked her up from grandma’s with the empty crate after the appointment. She asked “where kiki?”, and I told her kiki (Neville) had been very sick and had to go somewhere we couldn’t go with him. She let it go with that, and it didn’t come up again.
(My husband also found and adopted another cat within a week, which may have helped…)
Condolences on having to deal with so much in so little time