If DD decides to move out and is making sure that she has passports and photos, also make sure to take original birth certificates and marriage certificate so that she doesn't have to pay to get copies (which she will need). Also maybe all bank account details. Maybe even a wee withdrawal or two to have some cash before moving. Maybe half of any accounts with a credit balance - Am I going too far here ?? In case he tries to not tell all that he actually has when it comes to sorting/agreeing financial care for the children. Get details of any insurances.
Also, although DD may up sticks and leave, there is also the situation that he made it so unpleasant that she could not stay, i.e. she left for the sake of the children and for the benefit of her own mental health.
If it comes to the matter of custody, there can be joint custody even though the children live with mum. Joint custody means that father is entitled to details from schools, GP, etc. etc.
Depending on the age of the children, they have a say in what they want or don't want, including when it comes to daddy visits.