My daughter turns 3 in October. I want to make some figures for her cake from MMF, specifically an Alice and Cheshire Cat. Do I need to add gumtex or Tylose or will it harden without it?

Fondant will never harden w/o either which will help but won't set it HARD.  You could also add lot of cornstarch.  Maybe even need a combination of all three :)

Totally agree with kakeladi.   Marshmallow fondant is especially soft and pliable and without Tylose and/or cornstarch, not sturdy enough to work and it will never set up.