Maybe I'd add "and that others could see/touch/smell/hear/taste" - clarifying around vivid imagination, synesthesia, and that reality should be shared. It's the things all reasonable folks can agree about (given sufficient time and access).
Answers for little kids don't need to get too detailed and complicated. They'll develop nuance as they get older. Air feels and sounds like wind, though
It can help to ask in what context they thought of the question. For instance, what if they just want to know what makes "reality TV" different from regular TV?
It's like when the sitcom kid asks, "Mommy, where did I come from?" and after a whole birds-and-bees talk it turns out his new friend is from New Jersey.
In this case I had on the music video for the song "Transcendental Cha Cha Cha" which features several lyrics that mention the word "reality." Probably my kid was just asking the literal meaning of the word but they got distracted before we could discuss it further.
Reality is what is still there when no one is talking about it or looking at it. You can make ideas real and/or change them, but you can't stop reality by doing nothing.
The consistancy of our shared experiences and perceptions. The nutrients of a food is measurable and known without are reality, the tastyness of a praticular food is an opinion that may be mostly shared by our species but is not quantifiable or necessarily consistant. One person loves popcorn, another hates popcorn, but the amount of carbohydrates is agreed.