Are there other children going along? When my daughter was 10 yrs old her grandparents took her to Disney. She had fun but since she was the only child there it gave her more time to think about home,so she did miss us. Has she ever been to Disney?
Maybe if she helps in planning she will be excited enough that she won't get homesick. Also if she calls home everyday she can talk to her family and know they are still there,this will probably ease her fears . Are you close to your niece? If you are there is a less likely chance of her becoming homesick. Once in WDW most kids forget home anyway!