My thoughts on that as well LOL
That wouldn't make any sense though because why would the lady need to change everyone into child form except Dorothy when she said that the portrait was from a long time ago so she wanted to see the person as a child to check?
True but then again it's easier to get rid of children than adults.
She also said that she could have whatever she wanted to happen happen in that room, for example although Dorothy's body grew larger her magic was still the same. So even if the others had stayed as adults she could have just taken away anything that would have given them a fighting chance. I think the prince is going to end up being Toto.
What if the missing prince is actually a princess?? it would make sense with Dorothy mysterious past... #－.－)