When going through a divorce, it is best to be very considerate and respectful for the needs of the old family. It may be prudent to delay moving in with a new partner until the old family members have calmed down and got adapted their new roles.
It is always best to try to find solutions which everyone is satisfied with, so that children who feel abandoned will not get so desperately unhappy.
Mixing together the old and the new family may not work. Sometimes it is better that the man meets with his children and his new partner at separate times.
