If you want to have a character come to your kid’s party, there are many party entertainment companies eager to oblige. Look for local companies and call several months before you plan to have your event so you can ensure you get the booking when you want it. Characters mingle, take photos, run various activities, help serve the cake, etc. It depends on the particular character and company, so ask them what services they provide.
Keep in mind, due to copyright issues you’re not going to find a company with a Minnie Mouse. Instead, you might find someone offering a visit from “Girl Mouse,” “Mouse Friend,” or similar adjustments.