I would get either a Bearded Dragon or a Corn snake. Both good examples of hardy and forgiving reptiles that at least 9 times out of 10 will make great pets that wont bite or cause any real problems. They're both easy to handle and a joy to keep.
King snakes a good, but more chance of being bitey in my experience. Boas are great, ofcourse more attention is required and maybe not great as a starter snake with small children, however, some are unbelievably docile. I would get a corn first then consider a boa depending on how the family did with the corn.
I don't own any geckos and I think they are slightly more difficult to keep than Bearded Dragons, but I could be wrong... if I went for a gecko I would go for a Crested, but that's purely for looks. With children, I woudl suggest a Bearded Dragon still as they seem, imo, to be tougher... good for when it's handled with not so delicate fingers.