My brother-in-law was in a bad car accident in the summer, and his face required a lot of cosmetic surgery. His scarring is significant.
My two small children haven't seen him since then, and I'm nervous about holiday get-togethers. The kids are at an age where it's almost impossible to predict what they will think is worth mentioning (my older child mortified me by asking her dark-skinned teacher why her skin was so dirty). This is a very depressing time for my brother-in-law and I don't want to make things worse. Should I find a way to put off the kids' seeing him till they're a little older?
Answer: Treating him like a scary monster and keeping your kids away is not the way to help a loved one through a depressing time. That may not be your intent, but that's what you're doing.
Yes, it's impossible to filter kids completely. But it sounds as if both your kids and their uncle need each other right now. He could use some of the sweetness and natural acceptance that kids offer, and the kids could use a lesson in accepting differences in people.
To make their initial encounters go as smoothly as possible, tell your kids that their uncle looks different on the outside but is the same Uncle Frank on the inside. And warn your brother-in-law that your kids are going to be curious and might say out loud some things that adults never would. No doubt he knows that, but a reminder couldn't hurt.
Make sure you supervise and teach during their first couple of encounters - give factual answers to your kids' questions where warranted, or let him do it, and let them know gently when a question is impolite.