Hi there
What's the most simple way to describe what bitcoin is for people?
Will be cool to read in the comments some new creative ways to do that!

If you want to tell to them in a simply way and in way they can understand it well, just tell to them that it is a new form of money at the same time you can use it as an investment. But, I think 7 year old child can't understand it because their mindset is still in playing not doing money.
I would not say it to 7-year-old kids just because this is not yet for them. Even with fiat, most of kids out there still can't use it correctly, so there's no point to tell them about it yet. But if we want to tell to the adult, then the better comparasion will be gold since gold is well known all over the world. Bitcoin till today isn't allowed as a payment method here in my country, therefore most bitcoiners here will treat it as an asset instead of currency.