Yes, I think that most of Billy Crystal's movies are okay for children. He hasn't exactly done a lot of objectionable fare. Though I do wonder if kids would even understand his films like When Harry Met Sally. But his other work like Monsters Inc, Monsters University, and City Slickers are acceptable for kids.
Yes, I feel that Billy Crystal's movies are okay for kids, because most of his jokes and his style of comedy are okay for kids. The jokes that are adult themed would mostly go over the child's head. There is much worse content out there for children than the likes of Billy Crystal. He is okay for my children to watch.
None of the movies I have ever seen or heard him do a voice over in have anything that would be inappropriate for children. A lot of the movies he has been in have been voice overs in cartoons that are made for children. They all seem to be family friendly.
Billy Crystal has a long resume in Hollywood that involves a lot of the most popular films in the last few decades. I think a lot of his movies are actually okay for kids and families to watch together. But there are a few exceptions though where parents need to take into consideration.
The majority of Billy Crystal's films are family friendly. Just about all of them are rated PG-13. You may find a small amount of questionable language or humor, and possibly a bit of non-graphic violence. But nothing too vulgar, sexual, or offensive for most children ever occurs in a Billy Crystal film.