Hand lotion. I used Suave Advanced Therapy in the images. I’ve also used multiple other hand lotion brands, but I can’t remember their names now. We use whatever we have in the house and adjust the cornstarch amount as noted in the instructions.

As long as you’re playing with it when it’s out and put it in an airtight container when it’s not in use, it will stay soft. If you leave it our in the air for several hours, it will harden.
I would say it’s as easy to clean up as other playdoughs.

Each batch makes a little over a cup of dough. I regularly split one batch between my two kids. Depending on how much dough you want each kiddo to have, you could probably get away with 2 girls per batch.