I definitely think you should rent offsite. There's a list of offsite rental vendors in the disAbilities FAQ sticky in the stickies on the top of this board. Along with asking about one where she can keep her leg straight, at least part of the time, ask about a pediatric wheelchair. Standard rental wheelchairs are generally 18" wide which is too wide for most 11 year olds, even ones who may be tall enough (at 11, I was only about an inch shorter than my full height, but a 18" wheelchair would have been too wide). Even a 16" will make a difference with her comfort level. You can look at rental vendors near home as well as near WDW. If you're flying the wheelchair can be brought and will not count against the baggage allowance. If you rent at WDW and are flying, I'd definitely suggest requesting assistance with getting her to the gate. Depending on the airport this may be done using wheelchairs or through shuttles (most I've seen look like extended golf carts). Even smaller airports can feel huge if you're in pain while walking and Orlando's airport is huge.
If you do need to keep her leg extended, you're going to want a plan on how to protect that leg from being bumped by other park visitors. Just being in a wheelchair puts you out of the line of sight for most people. Having a leg extended out will be even more likely to be bumped because people don't expect manual wheelchairs to be that long. If possible, you'll want to use other members of the party to provide buffer space.
On the buses you will have a choice between folding the wheelchair and having her sit in a regular seat or having them strap the wheelchair down with her sitting in it. It's up to personal preference since all rental wheelchairs will fold in ways that allow them to be carried like strollers. If you're using DME, you might want to consider getting a note added to your reservation asking for a bus with a lift. Even if you're renting near WDW and getting the wheelchair delivered to your resort, the stairs are so steep that she'd probably be more comfortable using the lift to board/exit the bus.
At WDW she'll be able to take the wheelchair through most queues. For some rides she'll have the choice to also remain in the wheelchair, however, this will only work if the leg rests are in a more "normal" position. Most rides will require her to transfer and it's really going to be up to her to decide what she can ride (assuming she's tall enough for all the height limits). If she can't bend the knee at all, it's going to be worth considering rescheduling the trip or making it a trip focused more on shows and other activities. Some rides are also harder to board with knee problems. PotC is one that sometimes causes problems for people with knee problems due to the step down into the board. Test Track has an alternative boarding area that is a more straight across transfer so if she feels she can ride that other than the transfer (it's a pretty deep step down), she can ask about that.
