Hi Fazila,
You have a number of options for renting strollers for your granddaughters once you arrive at Walt Disney World.
Disney has single and double
strollers for rent at the entrance to each park and at Downtown Disney. Those are made of hard plastic, low to the ground, which makes climbing in and out easy for little ones. These strollers cannot leave the parks, although you can get a receipt and pick up another one without charge later in the day at any other park..
If you think you will want the strollers at your resort or will want to use them while traveling between parks, you might want to check into a couple of non-Disney companies that can provide a rental stroller for the length of your stay, such as Kingdom Strollers (www.kingdomstrollers.com) or Orlando Stroller Rentals (www.orlandostrollerrentals.com).
Those outside companies will deliver to your hotel.
Whether you choose to rent or bring your own, I strongly recommend that you have a stroller for your days in the parks. Even when my daughters were 5 or 6, we found that they had a much better time if they had the option of resting their feet occasionally in a buggy.
I hope that helps. Please let me or another of my Moms Panel friends know if we can answer any other questions. Enjoy your visit with your family!
Wilma