We took my oldest DD (now 7) when she was 3, and were initially disappointed that she was scared of the characters.
What we ended up with was taking pictures with a character where we were standing beside the character and she was being held by us on the other side of our body. Thus, we have pictures she can look at now with characters, but she still felt safe because she was a little distance away and Mom or Dad was holding her.
Interestingly, she suddenly the final day of our week there lost the fear and wanted to meet the characters, so we have lots of pics of her by herself with characters with her in the same outfit because it was all in one day.
So I guess my advice is similar to others; don't push it as it will likely backfire. Try our method of pictures with you if you like.
And similar to the other reply, make sure to try to have your child become familiar with as many of the characters you are likely to meet as possible. My second daughter (at not yet 3) had no fear meeting characters right out of the gate, but she was much more familiar with them than the first had been.
Nothing can change that Mickey will suddenly be 6'3", but watching some Mickey Mouse Clubhouse can't hurt...much...:animwink: