Wow...yooperfan...sorry if this offends you...but I hope the vibe you give in your post is not what you project to your niece. My parents were divorced when I was young and my Dad remarried, the woman he married is nice, but they never had kids together, and when I was a teen on vacation with them, I always felt like I was in their way....please don't do this to your niece....personally there is nothing I enjoy more than taking new people to experience Disney for the first time. Take it as a challenge to show her the best possible time...you'd be surprised how much better it would be for all of you.
OK, I apologize if I am misinterpreting your words and you are maybe just nervous about the experience...as for your question, it is not value season, I think regular season (We went in October last year, and in 2000, it is actually a great time to go, nice weather, pretty low crowds). The room size at the values are about 20 % smaller, not a huge difference (260 square feet for values, 315 Sq ft for mods). The bigger difference is amenities (mods have a fridge in the room, 2 sinks instead of one, and more options of things to do at the resort). So if your only concern is space in the room, I think you would be OK at the values.