To add to Woofgangs comment which I agree with - he may also be getting less attention now that there is a baby in the house (maybe your daughter had more time to fuss him when baby No1 was born, but not so much time after baby No2) and is therefore marking his territory to re-establish his position in the house.
My first thought would be to visit the vet and get him castrated if he is not already as dogs are far less likely to territorial mark when 'done'. A trip to the vet is probably necessary anyway to ensure he doesn't have a physical issue such as a bladder infection.
Could daughter get someone to come round to let him out or take him for walks if she is busy with the children and give him extra fusses/ do a bit of fun training with him outdoors during the day? That would help alleviate some of his stress.
Does he have a secure area/ room in which he stays when your daughter goes out? If so, could the children's toys be removed and his own toys be placed in there instead with his own scent on (less likely to pee on his own play-things).
If he doesn't have a bed, could she get one so he has his own designated area which is his own to feel secure in?
Does he get much less attention now than before?
Could she buy a plug-in diffuser to use to calm him?
Is he being kept out of certain areas of the house now and his territory has decreased significantly recently?
Obviously we can't have a fly on the wall view of why the dog is doing what he is, but dogs are fairly simple - if they were behaving very well at one point, but aren't now because OUR behaviour has changed. We need to change OUR behaviour again to correct it. It looks like the dog needs reassurance and a bit more time spent with him if possible (I know it must be hard, but worth the effort).
All the best