In answer to your question; Calmly and rationally. I know this will be extremely hard to do given the emotional impact of what is at stake. And you may not feel this way at the moment but I believe it will help keep defensiveness down so the conversation can stay on topic. Tell him you appreciate your MIL's concerns, but she does not know all the facts. Ask him to please not discuss your issues with his mom.
Then, be prepared. I have copied some information about Springer Spaniels from different sources. Perhaps learning a little more about the breed will help him feel better and go back to realizing there were some extenuating circumstances last night. Encourage him to look up the breed himself. (This will also give him ammo when his mother insists he do something about those awful dogs - you know she will!)
Next, do let him know how heart broken you would be if anything happened to the pups. Be sincere, but again not combative. They are companions for you while he is away. Tell him how hurt you feel that he would not trust you to keep the baby safe.
And lastly, in the spirit of compromise ask him for time. Let's see how things go for the next 4 weeks. If you still have concerns we'll talk about it then. My guess is after a month with no incidents it will be shifted to the back burner and finally forgotten.
These are only some opinions from a very outside observer. Hope they offer some help.
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English Springers are even-tempered, gentle, friendly, and sociable dogs that are great for kids. Intelligent, skillful, willing and obedient. Playful, energetic and a quick leaner. Merry, brave and sweet tail-waggers. Cheerful and courageous. It is affectionate, good natured and sincere, this dog loves everyone. Springers do best when they are with people as much as possible."
"English Springer Spaniel Temperament
The English Springer Spaniel is a very sweet and affectionate dog, with a gentle temper and pleasing disposition. Not inclined to fight with other dogs, he is generally considered the family class clown and tend to be an over-achiever, when looking for his master’s praise.
While many Spaniels tend to be ill-suited for homes with small children, more often than not, the English Springer Spaniel is very patient with the smaller members of the family and will endure far more ear-tugging and tail-pulling than other breeds. If anything, care should be taken with the Springer Spaniel, in the company of other dogs, so as to protect them from more aggressive animals. The English Springer Spaniel may be a capable field dog but, in truth,
he’s definitely a lover and not a fighter."
"Temperament
The typical English Springer Spaniel is a happy-go-lucky and eager to please dog. They should be friendly, sociable, playful, and gentle,
making them an excellent family dog. They are extremely intelligent and can learn very quickly."