A staple of childhood games, playing house (a broad term for "playing pretend in the real world") has long been a way for youngsters to start experimenting with roles they may take on in later life -- parents, students, shopkeepers, actors, firemen and women, and anything else they might be interested in over the course of their maturing years. Indulging in this game with your children can be a great chance for you to turn the clock back and play the what-if game, while teaching your youngsters fundamental values they can use in their futures.
Each take on a role, something you aren't in real life. Don't play the parent, or you'll just spoil the fun of the game for your children. If you want to step back and let your children take control, you could be the store owner, to whom they come when they need supplies (but otherwise, the game is in their hands). If you want to be more interactive, you can join in as a role you don't normally play in your everyday existence. It's escapist for you and freeing for them!
Use props, things that are durable and inexpensive. There are tons of plastic "playing house" toys you can buy for your children, and while these are great if you can get them secondhand or borrow them from friends (think of the value of a toybox swap!), you can also use real props as long as you choose ones that won't hurt your children if they break. For example, it might be cool for them to have real electronics, but if there is any danger of getting shocked, you should avoid them.
Come up with unique ways to play. If you have a window near the ground, you can make it a drive-thru restaurant, for example. Or set up one room as a vet's office, with someone taking on the role of the vet and someone the pet owner.
Use playing house as an educational opportunity. Here's just one example: If one of your children chooses to be the drive-thru window operator, for instance, you can teach them math skills. Create a menu that has whole dollar amounts for each item, and then you or one of your other children orders what they want. The cashier then must correctly do the math on a calculator or scratch paper. It's a fun way to combine play and learning!
Gather up some inexpensive playing house supplies at garage sales! There's always a cheap plate or cup or big coat that your children will love to use as a prop.