You know your own family and expect them to be different once you are there. It sounds from what you say that they are a 'normal', stressed-out family, with loads going on in their busy lives. Add new baby and home improvements and I'd suspect the children are probably repeating the short shrift 'you're interrupting me' from those stressed out parents. Perhaps when you usually visit they expend quite a lot of energy looking after you. This is a main complaint from friends of ours who now live abroad, that visitors from UK forget they have to go to work and also expect to be waited on/given the tour/paid for etc. Not to mention the increased laundry, cooking and so on.
Having said all that, I think you should go. What your DS has said is only that he can't put you up - quite reasonably given the circumstances. So don't expect to stay with them AT ALL and book yourself nearby accommodation for the full stay. Insist on getting yourself to and from the airport. If he's a busy chef they may live in a city in Canada so there should be plenty to do. Your existing DGC are only 6 and 7. Investigate fun places you can take them out for the day to leave the new third-time parents alone for a bit. If you have the finances and can arrange it, let them have a 'sleepover' with you. Rebuild your relationship with the children and concentrate on letting them enjoy some relaxed family time again with you. Take lots of pictures to remind them of the fun time the 3 of you had. Christmas is a good time to visit because the parents should get a least a tiny bit of leave, if only Christmas Day, and you can call in a few times briefly during the break with lots of food, tidy/wash up a bit if required, admire new DGC ... and then leave. Maybe you could take them out for Christmas lunch, for a Christmas present, or cook it all for them, to give your DS a break from the kitchen? You alone know what will be helpful and what you can afford. In the end, if all is as horrendous as you fear, prepare some things you can do mostly alone and look upon it as a brief visit to them to see 3 lovely DGC and an extended holiday for you. At least you will have made the effort to see them without putting them to any trouble. But, if you know you wouldn't want to look after yourself, and your visit will create a lot of work for them, then don't go yet.
Lack of public toilets in towns.