How do I tell my neighbour the dog barking is unbearable without becoming enemies

Long story short: family of 4, moved in a couple of months ago, terraced house. My neighbours have two small dogs, one of which whenever they’re away barks incessantly until they return. A lot of times in the evenings when presumably they’re out for dinner etc the dog barks non stop. I messaged them once politely asking if the dog was ok or needed anything, to which they replied “oh no, it must be scared to be alone we’ll be back in 10min so sorry.”

Then a second time after it was barking for over 1h30 I messaged again asking what was going on and they said “so sorry we’re at the cinema, it just seems to bark when we’re away, we’re going to try training soon”

Now the dog is barking again for over 1h… I don’t want to be the annoying new neighbour. I actually love dogs, and would love to have one, one day. But I just don’t understand how you can be so inconsiderate as a dog owner. How do I navigate this without making things hostile?