Hey there from a fellow foster parent!

I think what people don't understand are the
so many different situations you can have with your foster children. For those that don't know a lot about the process, it can get complicated. The process varies by state, but typically a child goes into foster care and their bio parents are given a "plan" by the court to get their kids back. They are usually given a year or two to complete the plan, which can include parenting classes, drug treatment, anger management, lots of things. It alllll depends on the situation. The child could be in any point in this process when you get them. Just removed, bounced around, just removed from a foster parent's house they really liked, close to going back to their parents, etc. They could be babies to teenagers. Sometimes they have siblings in other foster homes. Sometimes their bio parents have visits, sometimes they don't. Sometimes the visits are overnight, sometimes they're just an hour supervised. Sometimes you know right away the child won't be reunited with their parents because maybe they have died or are in jail. So while they are in your care the state looks for relatives to take the child or for an adoptive family. Sometimes they want you to adopt the child. Sometimes the children are young and loving and fit in, sometimes they have special needs, sometimes they are older and hate you lol. Sometimes you and/or the child are excited they are being reunited with their parents, sometimes one or both of you know it's the wrong thing but can't stop it. This is just the tip of the iceberg, but all of these things effect your family dynamic, and can change on a dime. The whole point in all of this is just to say that her issue of taking the child on a trip isn't as black and white as it seems. When you first get into it you have this idealized attitude and think that you would
always take your foster kids on trips and they will
always be just exactly like one of your own children and you are going to change their life and they will adore you for it. But real life situations will quickly bring you into reality. This is not to say it's not worth it because it absolutely is, it's just not so cut and dry when it comes to these kinds of decisions.
And honestly when the child is that close to going back, you have to think about the other families feelings as well. If you were just about to get your child back after fixing your life up (hopefully), would you want strangers taking them out of state and showing them the best time of their entire life? Wouldn't you be worried that either they could get hurt or they wouldn't want to come back to you? There are just soooooooo many issues involved.
Now my "advice" if you can call it that lol. We did take our 3yo foster daughter to WDW last year, BUT our situation was totally different than yours. We don't have any other children, and we were half way to adopting her. In fact she is adopted now

In your situation, I wouldn't take her for several reasons. Your family does need time to bond, especially if you foster a lot. This child is about to return to their family, and depending on the child this could make things difficult for them later on. She might expect her bio family to take her on trips and have high expectations they could never fulfill. And honestly, she probably doesn't have a concept yet of what DisneyWorld even is at her age, even if she's heard of it. I doubt she would even know what she was missing. I would ask your children not to gush about it in front of her and wait to put pictures up until after she leaves. Make sure whatever she does do while you're gone is super fun. Sleepover possibly? Respite with a family that has kids her age?
On the other hand if you just adore her and want to take her, that's ok too! Talk about it with her case worker(s) and possibly any bio family you have contact with. But don't feel guilty about taking care of your own family. Your own biological children make sacrifices in many ways when you foster, and this is a great way to show them that you love them. IMO it would be different if this was like a 5+ year old child that you had just gotten or something. Then I think that child would feel bad and left out, but honestly this child won't know what she's missing. She will probably grow up and never realize it even happened. If it gets closer and you find out that you will have her even longer then you might consider changing your mind, but as-is, it sounds like you have a clear head on the matter, and a big heart to boot, which is a good thing

Big hugs to you and thanks for all you do!
xoxo
Val