@papa4713 Sounds like you'll all be learning...what a great thing to do with them! My honest opinion is...start at home. Yours, theirs...doesn't matter. Bury some coins and some trash (wads of aluminum foil, soda can pull tabs, nails...stuff you'd be likely to run into). I wouldn't pretend it was all "already there", because i wouldn't want to set a unrealistic expectation that every hunt will make them rich. But let them know it's a practice run to see what the machine can do.
Chances are that there will be some naturally occurring items in the lawn already, anyway. But the most important aspect of hitting your own private property first is to learn how to replace the dirt and grass as though you were never there.
I practiced on my own yard and friends yards for weeks before I hit any public places. Leaving holes, loose dirt, and dead vegetation is a bad look for the hobby at best, and can get public places restricted from metal detecting at worst. So just take the time to learn how to fill holes and cover your tracks.
Beaches are also a good idea, since it's really hard to make a mess in sand. Also, playgrounds that use wood chips and the like...really easy to smooth it all out after getting a target out.
Once you've gotten good at leaving no trace of your activity, elementary school grounds are usually good. School kids drop a lot of change. Also, little league baseball fields, where spectators sit...especially where younger siblings roll down hills or wrestle in the grass while waiting for older siblings to finish their games.
There are probably a bunch of good YouTube videos on how to learn to retrieve targets responsibly. As well as some posts on this forum.
It's a great hobby and a ton of fun for kids.