Brian,
Finding Mulan can be a bit of a roll of the dice, but Mushu is, in his own way, a sure bet.
Let's start with Mushu. While I do not know of any character meet-and-greet opportunities with him at any of the parks, he is part of the attraction at the
Magic of Disney Animation located at Hollywood Studios. He is a cartoon in the opening section of the attraction, and is, as always, pretty funny.
Mulan is a bit harder. She can be, on occasion, one of the princesses in attendance at meals at the
Akershus Royal Banquet Hall. They do not pre-post which princesses will be in attendance there, so she might be there, she might not. There is no way to be certain.
Mulan can also, on occasion, be seen at the China pavilion in Epcot. The best thing you can do is ask a cast member (at any park) if they can find out if there is a scheduled character meet-and-greet with Mulan that day. They have a central office they can call to find out.
Hopefully your kids will get to meet them!
Have a magical vacation!