Tonight with your 5 to 8 year old
The prompt
“Has there ever been a time you felt happy and sad at the exact same time? What was that like?”
What to listen for
If they can describe a real example (leaving a fun place, a friend moving, finishing something they loved), they're developing real emotional range, the ability to hold two feelings at once is a big, mature step.
If they seem confused by the question or insist feelings only come one at a time, that's completely normal at this age, you can offer your own example ("I feel that at the end of vacations") to plant the idea gently.
If they say "I don't know," leave it be, this is a big concept and just hearing you talk about it plants a seed for later.
What this tells you
This shows how ready they are to understand that feelings can be complicated and mixed, not just one thing at a time.