This is a tricky issue. Depending on how old your children are, you could try explaining that because your father-in-law got sick, his language abilities are similar to those of
a younger child, such as a toddler.
You might say something like, "Grandpa's stroke made it so he can't talk the way he used to. It's kind of how it is for your baby brother: He can't understand everything you say, and it's hard for him to talk. The main difference is that we can't know for sure when Grandpa will be able to talk again."
This comparison may help your children understand that their grandfather can't speak as well as a healthy adult and that he needs help with communication. When you put it this way, they may be able to relate to his difficulties.