Welcome to GJ, Twinsmom. My twin girls just turned 13 last week. Man, how time flies... Enjoy yours while they grow.
To give some ideas for your specific case, can you give some more info on what you have to start with? Do you have a ceiling or just open rafters? What are the walls now?
Ive seen garages turned to spare bedrooms and offices with just painting the drywall walls, drywall ceiling and carpet. If there is a water heater or A/C unit in the garage, some doors to enclose will make them visually go away. At the front of the garage, I've seen a new drywall partition installed a couple feet inside the roll-up door but only high enough to allow the door to still operate. The couple feet on the door side could be additional storage or shelves. But, whatwver you do, by all means make it removable to get the garage/workshop space back when the kids grow up...
Good Luck with your project and your twins,
jeff