The EASY question to answer is where guests using wheelchairs wait for attractions.
Even though people think most attractions have 'wheelchair' lines, most attractions have lines that are wheelchair accessible, so there is no special wheelchair line.
Post 11 of the disABILITIES FAQs thread lists the 'Mobility Entrances' for attractions and which attractions require a transfer from a wheelchair for the attraction. There are some attractions where the line is accessible, but there is a different boarding and/or unload area for guests with wheelchairs.
With an obvious leg cast, you would be able to bring a stroller into lines with you where strollers are not usually allowed.
I'm answering the rest of the questions as an RN and a mom whose child had 2 long leg casts connected by a spreader bar for 6 weeks after surgery.
The next easy question is whether you should carry her.
No.
A 3 year old would be hard enough to carry in lines. A 3 yr old in a cast is going to be very heavy, and with the cast, the weight won't be balanced, so you will want to pick her up as little as possible. You will also want to avoid walking with her as much as possible because it will be very awkward and you could fall.
Do they allow casts on attractions is harder to answer.
All shows have places for wheelchairs, so you will have no problem with bringing a wheelchair/stroller as wheelchair into shows.
Most 3 year olds won't be riding very wild rides and even the wilder rides allow casts as long as they don't interfere with the restraint system or move around in a way that they could hit other guests.
The question will be what kind of cast she has and whether or not she will fiot into the attraction. Depending on the type of cast, she may not be able to sit or may not be able to sit on a ride seat, since they are generally not very deep and are made of slippery plastic.
Stroller, special needs stroller or wheelchair?
That will depend on what kind of cast she has. Obviously, as a nurse, I can't answer medical questions, just nursing ones. I don't know how bad th break is or what kind of cast her break will require, so I'm just giving general information about femur fractures.
In most fractures, they try to immobilize the joint above and the joint below the break.
For the femur, the joint above would be the hip and the joint below would be the knee and/or ankle.
In a best case scenario, they might be able to cast her from just where the hip joins the trunk to the toes. That would be difficult, but she could still sit. You would need to carefully choose a stroller or wheelchair that would support the cast. Your best bet would be to work with an equipment rental place near your home since you will almost certainly need something to get around in your home and neighborhood while she is in a cast.
You could bring the equipment with you - mobility devices can be gate checked on the plane with no charge and not counting against your bag allowance.
On a plane, you may need the bulkhead seats for more leg room.
The worst case scenario would be something called a spica cast, which starts on the chest or lower chest and continues down the broken leg all the ways to the toes and halfway down the other leg ( about to the knee). There is usually a bar that goes between the 2 leg casts to keep everything very stable. This is the most likely type of cast for a preschool child with a broken femur from what I know.
This website has a good explanation of a spica cast with pictures (the website is a bout femur fractures in children).
http://www.hss.edu/conditions_femur-fractures-in-children-broken-thigh.asp
If she has a cast like that, she will have a hard time sitting and may not fit on an airplane at all. You would also need a very modified mobility device for her.
My youngest DD was in that sort of cast after surgery and for the first few weeks, she could only be in a wagon with lots of pillows. Then her cast was cut down and she could sit ( although her long leg casts were still stuck together with the spreader bar). At that point, she could sit in her own wheelchair, but we had to modify it with a wedge shaped piece of plywood, well padded, to support her legs. That made the wheelchair very wide where her feet were, so it didn't fit a lot of places.
Another issue would be transportation. DD didn't really go anywhere while she had the big cast ( other than home from the hospital). Once it had been cut down, we did take her in our van, but we took out the middle seat to make more room and rigged up some milk crates connected together to support her legs. We didn't have a wheelchair accessible van, but she could go to school in her wheelchair in the accessible van.
If you do go and have a stroller, she would need to be removed from the stroller and it would need to be folded for use on the buses. You could take her onto the bus in the stroller, but then would have to remove her and hold her on a bus seat.
The last thing would be comfort. A long leg cast will be hot; a spica cast will be hotter, plus you will need to deal with 'sanitation' since much of her lower trunk ( near the 'business ends' will be covered).
Personally, I would seriously consider not going if she has a long leg cast and I would not go at all with a spica cast.