I got a pup last year, and, yep, he barks sometimes. Not all day, thankfully, because, I would go nuts myself! So, the first question is - Are the pups REALLY barking all day, constantly, or are they barking for short periods throughout the day? If it is the former, the owners are probably going crazy themselves! If it is the latter, it is probably just something you will have to chalk up to London living. Much like children, they do make noise, unfortunately. And, at this time, much like children, dogs are a bit confused and bewildered about what is going on. I can hear children squealing and screaming all day at the moment, too, and, to be honest, it is probably far easier to train them to be silent than it is to train a puppy to be quiet! However, I wouldn't ask them to. As long as parents of humans and dogs attempt to quieten them at nighttime, then I accept that daytime in London IS noisy. Even during lockdown, without the constant traffic and construction work that normally drowns out every other sound!
We have had problems with our neighbour recently, after they shoved anonymous shirty notes through the door when I had to leave my pup home alone for a couple of hours for an unavoidable appointment (prior to lockdown, obviously, and the first time in over six months of having him), when I am normally with him all the time. I can say with 100% certainty that that is NOT the way to deal with it! Even after I put my own note through several neighbours' doors apologising, explaining the situation, and offering to speak to anyone who had been bothered by his barking about the steps I have been taking to stop it, they STILL put more abusive notes through the door, which resulted in having to get the police involved in the end. And the police told them to stop, because what they were doing was harassment. So, if you do decide to approach your neighbours, definitely don't do it that way!
As for training, yes and no. Firstly, if you are asking can they get a professional trainer to help, maybe they can, but not at this time, and unless you are offering to pay, I personally wouldn't pay for a professional myself. Secondly, training a dog not to bark can actually involve more barking to begin with, because if they are doing it for attention, then immediately trying to quieten them simply results in it becoming an ingrained attention seeking behaviour. So, sometimes, I have to ignore my pup when he is barking. Not easy, for myself or my neighbours, but the more I pander to it, the more he barks! Your neighbours may be doing this. Thirdly, it can be anxiety based - his anxiety & mine. And, needless to say, I am anxious at the moment! Being left alone can cause him to be anxious, so I was trying to build this up gradually, but, almost a year on, we keep having issues which set us back to the beginning (he has had several health problems, and now we have those sorted, we are now in lockdown). Changes to his routine also make him anxious, and cause him to bark more. Lastly, some of his barking can also be totally random, though - he will bark at his water bowl if he deems his water too dirty to drink, or chat to his chews, or if he sees a certain shadow he will shout at it, or an ant, or if he wants to play he will yap at us, or if he wants to talk back to me when I have told him off (he's in the "teen" phase!). Oh, and if he wants whatever I am eating, he will bark at me in the hope I will hand over my meal! All in all, whilst I do try (and I am sure your neighbours are trying, too, especially if they are home at the moment as well? If not, is it possible they are keyworkers who would normally use doggy daycare which is now shut?), it isn't possible to stop all barking. Nor do I want to make him completely silent, to be honest, because he is our security - I WANT him to bark if we are ever in danger! That is one of the reasons why we got him.
A bit of a long answer, sorry! To summarise, I would say that unless the dogs are barking at night, you may have to be a little more accepting of daytime noise, especially in these exceptional times, as things are out of whack for everyone - human and canine alike. If you do decide to speak to the neighbour, do it in person and be friendly about it. It's really difficult when we are all cooped up at home far more than normal, though, and can hear noises, or witness behaviour, that weren't a problem before, so I do understand it from your point of view, too, but I wouldn't preempt problems for the summer just yet. Hopefully we will all have the option to go out and escape the confines of our own homes by then! I know we will be out and about with our hairy beast as much as possible as soon as we are allowed to! Having only one walk a day is driving us all a little cuckoo!