Fat tire vs regular mountain bike in mud
Mud is even difficult for experienced riders. And a fat tires is not always better at mud.  If you need to dig down to get to solid ground a more narrow tire can be better.  

Sand is where a fat tire is nice.  But even soft deep sand is going to be a challenge on a fat tire.  

I would look for a 29" mountain bike that will take up to 2.4" tires. 
And there are are mud tires.  Mud tires does not just get better traction but they shed the mud better.