As they all sound lovely people I think it would be good if you talk to each one in turn, in confidence telling them your fears. Ask your fil or dh (or you yourself!) get a crate for the dog and or put the dog in the car. Then let your mil know what you have asked for and keep ringing up and checking with all of them that it is in place before any visit.
You could also asked for the dog to go to the kennels of stay with a friend for every visit or even offer to have the dog yourself! Anything to ensure they take your concern seriously.
Really you need to ask them for a long term plan regarding the dog and your visits, but don't whatever you do just leave it up to your fil to deal with it as his past totally irresponsible actions should have your red warning lights going off in every direction.
It is not just now you need to worry about but as your dc gets older. The dog is unsuitable to be around anyone small and vulnerable and your mil sounds as if she would back you up on this.
As a dog owner myself, I find it amazing that lovely people who are owners of really big, powerful, barking loudly and lively dogs say 'oh he just wants to play/be friendly/say hello' when the dog has just jumped up/scratched/ left teethmarks/knocked you or your dc over!
Ds has a friend who has a large, powerful young dog and his dm doesn't teach it not to jump up and scratch as she thinks it's just being friendly 